TRPV1

In brief

What it is: a pore that answers to heat, acid and pepper

TRPV1 is an ion channel. That means a protein that spans the membrane of a cell and forms a hole through it, a hole that is normally shut and that opens when something specific happens. When TRPV1 opens, positively charged ions flow into the nerve cell, calcium among them, the inside of the cell becomes less negative, and if the change is large enough the cell fires an electrical impulse toward the spinal cord ¹. The cells that carry it are nociceptors: the thin, slow sensory fibers whose job is to report tissue damage, which is what the brain renders as pain ⁷.

It was found by looking for the chili receptor. Capsaicin, the pungent compound in hot peppers, had been known for a century to excite and then silence a specific class of sensory nerves, and the hypothetical membrane site it acted on was called the vanilloid receptor, after the vanillyl chemical group that capsaicin and its relatives share ^6,26. In 1997 Michael Caterina, David Julius and colleagues screened a library of complementary DNA from sensory neurons for any clone that would make a cell take up calcium when capsaicin was applied, found one, and reported that the protein it encoded was a non-selective cation channel related to the transient receptor potential family of membrane pores, abbreviated TRP, and that it opened not only to capsaicin but to temperature in the noxious range ¹.

The structure stayed invisible for another sixteen years. In 2013 Maofu Liao, Erhu Cao, David Julius and Yifan Cheng used electron cryo-microscopy, a method that freezes purified protein and images it with electrons rather than crystallizing it, to solve TRPV1 at 3.4 angstroms, fine enough to see individual amino acid side chains, and the first membrane protein solved at that resolution without crystals ³. The picture is four identical subunits arranged with four-fold symmetry around one central ion pathway, built from transmembrane segments 5 and 6 and the pore loop between them, each flanked by a bundle of segments 1 to 4 that looks like the voltage sensor of a voltage-gated channel. The outer mouth is wide, the selectivity filter is short, a conserved stretch called the TRP domain presses against the linker between segments 4 and 5, and the subunits are held together at the bottom by ankyrin repeats, a common protein-binding motif, inside the cell ³.

A companion paper caught the channel in two open states, using a spider peptide toxin and small vanilloid agonists to hold it there. Two findings mattered. The segment 1 to 4 bundle, which swings during the activation of a voltage-gated channel, stays put in TRPV1, so the family resemblance is anatomical rather than mechanical. And the channel has two gates in series, one at the outer pore and one at a narrow greasy constriction below it, both of which widen on opening; the authors proposed that allosteric coupling between them accounts for how many different things can open this one channel ²⁷.

The whole history, including every reversal

The history of TRPV1 is unusual in that the pharmacology arrived first and was correct, the gene arrived second and was celebrated, and then two of the field's central claims had to be withdrawn: that TRPV1 is the heat sensor, and that blocking it would make a painkiller.

By the middle 1990s the vanilloid receptor was a well-characterized pharmacological object with no known molecular identity. Arpad Szallasi and Peter Blumberg could describe its ligands, its restriction to primary sensory neurons involved in nociception and neurogenic inflammation, and the therapeutic logic of desensitizing it; they could also say, correctly, that the irritancy of capsaicin was the main obstacle to using it, and that the search was on for analogs with a better ratio of desensitization to burning ⁶. Their 1999 review in Pharmacological Reviews was the standard account of the receptor at the moment the gene was published ²⁶.

The reversals since have been of two different kinds. The first is a narrowing: TRPV1 turned out to be one of several heat sensors rather than the heat sensor, a correction that began in the knockout papers of 2000 and was completed in 2018. The second is a failure of a drug class: the entire first generation of TRPV1 antagonists was stopped because blocking the channel raises body temperature, an effect that was predicted by nobody and that turned out to be the channel's day job ^10,31. The 2021 Nobel Prize was awarded for the discovery, not for a drug; the drug that exists is a patch of pepper extract ^9,12.

What opens it: pepper, heat, acid, and the body's own lipids

Capsaicin is the defining key. It is the pungent principle of hot chili peppers, and it opens TRPV1 by binding a pocket on the inner face of the channel; the original cloning showed that expressing this one protein in a cell that had never been sensitive to capsaicin made it sensitive ¹. Resiniferatoxin, a diterpene from the North African plant Euphorbia resinifera, is the same kind of key cut far deeper: it is described as the most potent known agonist of the channel, and the concentrations needed are correspondingly tiny ^6,34.

Heat opens it directly rather than through some intermediate. Tominaga and colleagues recorded heat-evoked currents through single channels in membrane patches excised from the cell, where no cytoplasmic machinery remains, which is the clean demonstration that temperature gates the protein itself ⁷. The threshold quoted everywhere, above about 43 degrees Celsius, comes from this early work and is the temperature at which skin heating starts to hurt ⁴.

Protons, meaning acid, do two things. Moderate acidity lowers the heat threshold, so that at a pH of 5.9 or below the channel opens at room temperature with no added heat; this is why injured, inflamed or ischemic tissue, which is acidic, hurts at temperatures that are otherwise comfortable ⁷. Sven-Eric Jordt and colleagues then separated the two actions at the level of single amino acids. A glutamate at position 600, in the stretch linking the fifth transmembrane segment to the pore, sets how strongly protons potentiate the channel's response to everything else. A second glutamate, at position 648, is required for protons to open the channel on their own, and mutating it leaves the responses to capsaicin and heat intact ²⁵. Stimulus-specific routes into the same gate is the structural claim the 2013 dual-gate model later made sense of ²⁷.

The body makes its own agonists, and the evidence for them is uneven. Anandamide, an endogenous cannabinoid, dilates arteries in a way that is capsaicin-sensitive, releases calcitonin gene-related peptide from perivascular sensory nerves, is blocked by the vanilloid antagonist capsazepine but not by a CB1 blocker, and produces capsazepine-sensitive currents in cells expressing the cloned receptor; other endogenous and synthetic cannabinoid agonists did not reproduce the effect, which argues the action is at the vanilloid receptor rather than at a cannabinoid receptor ⁵. Work on the pharmacology of anandamide transport then showed that several compounds designed as transport inhibitors, including AM404, arvanil, olvanil and linvanil, activate the human receptor at concentrations 400 to 10,000-fold lower than those needed to block transport, which is a warning about attributing any effect of these tools to the transporter alone ²⁸. A 2026 review summarizes the current framing: anandamide behaves as a dual agonist at CB1 and TRPV1, and TRPV1 activation engages protein kinase C, protein kinase A and mitogen-activated protein kinase pathways that feed back onto the channel's own phosphorylation and the neuron's excitability ³⁵. These are isolated-tissue, cell and modeling data; what concentration of anandamide reaches TRPV1 in an intact human, and when, is not settled.

Sensitization is the fifth key and the most clinically important one. Inflammation does not simply open the channel; it lowers the threshold at which everything else opens it, through the kinase pathways above and through the acidity of injured tissue ^7,35. The clearest in vivo evidence is negative: mice without the channel develop almost no heat tenderness after inflammation, in one study and completely none in another ^4,22.

The correction: TRPV1 is not the heat sensor

The 1997 paper made a careful claim, that the channel functions as a transducer of painful thermal stimuli, and the field rounded it up to something stronger ¹. The knockouts, three years later, began the correction from both directions at once. Caterina and colleagues found impaired detection of painful heat ⁴. Davis and colleagues, with a separately made line, found the animals behaved normally in response to acute noxious thermal stimuli and wrote that alternative mechanisms are sufficient for normal sensation of noxious heat ²². Both papers agreed that inflammatory heat tenderness depended on the channel, and that acute heat sensation largely did not.

The resolution took eighteen more years. In 2018 Vandewauw and colleagues built mice missing TRPV1, TRPM3 and TRPA1 in all combinations and showed that robust heat responsiveness, measured both in isolated sensory neurons and in rapidly firing C and A-delta nerve fibers innervating the skin, survives as long as any one of the three is functional. Only the triple knockout lacks the acute withdrawal from noxious heat that prevents a burn, and those animals still sense cold and mechanical stimuli normally and still prefer moderate temperatures ²³.

This matters beyond tidiness. It explains why blocking TRPV1 alone was never going to abolish heat pain, and it predicts the thermal hypesthesia, meaning reduced heat sensation, that has limited antagonist development ³⁶. It also means that the burn risk from a defunctionalized patch of skin is partial rather than total, which is consistent with the human data: after a single 8 percent patch application, heat- and cold-detection thresholds did not change significantly at any timepoint over 24 weeks, even while 80 percent of the nerve fibers were gone ¹¹.

Why capsaicin burns and then numbs, and the 8 percent patch

The paradox is old and the explanation is modern. Capsaicin hurts because it opens TRPV1 on nociceptors; then it stops hurting, and then the skin it was applied to stops reporting pain at all for weeks. For decades this was attributed to depletion of substance P, a pain-related peptide, from the nerve endings. Praveen Anand and Keith Bley argued in 2011 that substance P depletion is only a correlate of capsaicin treatment with little if any causative role, and that the real process is better described as defunctionalization: the terminal loses its membrane potential, becomes unable to transport neurotrophic factors, changes phenotype, and reversibly retracts its epidermal and dermal endings ¹². That reframing is the current standard account, and it is itself a quiet reversal of the textbook mechanism.

The human measurements are clean because they were done in healthy volunteers rather than patients. Kennedy and colleagues applied a single 60-minute high-concentration patch to the thighs of volunteers and took skin biopsies. At one week, epidermal nerve fiber density had fallen by about 80 percent compared with unexposed sites; tactile thresholds rose by about 8 percent and the proportion of stimuli reported as sharp mechanical pain fell by about 15 percentage points. At 12 weeks, regeneration was evident but incomplete and the sensory measures had returned to baseline; at 24 weeks, recovery of fibers was about 93 percent. Heat- and cold-detection thresholds did not change significantly at any timepoint ¹¹. The fibers come back. That is the whole safety argument for the patch.

The efficacy is real and modest, and the honest way to express it is with the number needed to treat: how many people must be given the drug for one additional person to get the benefit who would not have got it on placebo. The 2013 Cochrane review pooled six studies and 2,073 participants, all using a 0.04 percent capsaicin patch as the control to preserve blinding. In four postherpetic neuralgia studies with 1,272 participants, the number needed to treat for a patient to report being much or very much better was 8.8, with a 95 percent confidence interval of 5.3 to 26, at eight weeks, and 7.0, interval 4.6 to 15, at twelve weeks. In painful HIV neuropathy, two studies and 801 participants, the corresponding number at twelve weeks in a single study was 5.8, interval 3.8 to 12. Serious adverse events were no more common on active treatment, 4.1 percent, than on control, 3.2 percent ³³. The earlier 2009 version of the same review had found a number needed to treat of 6.6, interval 4.1 to 17, for repeated low-concentration 0.075 percent cream across six studies and 389 participants, with a number needed to harm of 2.5 for local skin reactions ³⁷.

Not every trial worked. A multicenter study randomized 155 patients with postherpetic neuralgia two to one and missed its primary endpoint outright: a 36.5 percent mean reduction in pain on active treatment against 29.9 percent on control, p equals 0.296. Only in a post hoc subgroup of patients whose neuralgia had lasted at least six months did the difference reach significance, 37.6 against 23.4 percent, p equals 0.0291. The authors attributed the failure to spontaneous resolution in the recently affected control patients ³⁸. Post hoc subgroups are hypothesis, not result, and the paper says so.

Safety over a year has been tested in the condition where the patch is most used. An open-label phase 3 study randomized 468 patients with painful diabetic peripheral neuropathy to repeat patch treatment of 30 or 60 minutes plus standard care, or standard care alone, for 52 weeks. Quality-of-life scores improved more with the patch, neurological examination scores did not deteriorate, and sensory testing of sharp, warm, cold and vibration stimuli showed no worsening; application site pain occurred in 28.2 and 29.3 percent of the two patch groups ³⁹. The study was conducted and authored by Astellas, which marketed the product in Europe, and that should be read into it.

Where it stands in 2026. In the United States, capsaicin is among the agents approved for painful diabetic neuropathy, restricted to the feet ¹³. A 2025 network meta-analysis of 38 randomized trials and 8,621 participants in postherpetic neuralgia placed the 8 percent patch among the most effective single agents for pain intensity, with a mean difference of minus 9.20, interval minus 12.0 to minus 6.60, though the combination of amitriptyline and pregabalin ranked highest overall and 24 percent of included trials were at high risk of bias ⁴⁰. A 2026 systematic review of long-term evidence across peripheral neuropathic pain found the patch had been studied in more long-term studies than any other recommended treatment, thirteen of them, with the lowest rate of discontinuation for adverse events; the review was written by employees of Grünenthal, which markets the patch, and that is disclosed in the affiliations ⁴¹. And the limits are real: a 2025 systematic review looking specifically at chronic postoperative neuropathic pain screened 487 studies, found exactly one randomized trial with 46 participants, and that trial was negative, with the certainty of evidence graded very low ⁴².

The reversal that defined the field: the antagonists and their fevers

If opening the channel hurts, closing it should relieve pain. Several major pharmaceutical companies drew that conclusion, and for several years the preclinical data were encouraging ^29,31,43. Amgen's AMG 517 blocked capsaicin, proton and heat activation of the human and rat channel completely and potently, and blocked capsaicin-induced pain behavior in rats ²⁹.

The problem showed up in the toxicology package. Transient increases in body temperature appeared in rodents, dogs and monkeys. The team investigated rather than ignored it: acetaminophen suppressed the hyperthermia, and repeated dosing attenuated it while the analgesic effect persisted, so they concluded the effect might be manageable and took the drug into people ²⁹.

It was not manageable. In phase 1, TRPV1 blockade produced marked, reversible, generally plasma-concentration-dependent hyperthermia; the tolerance seen in animals appeared again at the highest dose; and then, when AMG 517 was given after molar extraction as a test of analgesia in acute surgical pain, it produced long-lasting hyperthermia with a maximum body temperature above 40 degrees Celsius. The mechanism was worked out in rats in the same paper: the drug constricted tail skin vessels and increased thermogenesis, meaning the channel normally restrains both. The conclusions were stated without hedging: selective antagonists like AMG 517 cannot be developed for systemic use as stand-alone agents; individual susceptibility determines how high the temperature goes; and TRPV1 is a pivotal regulator of body temperature in humans ¹⁰. A fever above 40 degrees Celsius in a trial of a painkiller for toothache is the kind of result that closes a program.

Two obvious escapes were tried and failed. Keeping the drug out of the brain did not work: a series of peripherally restricted analogs still raised core temperature at doses that covered the target, which placed the thermoregulatory site of action outside the blood-brain barrier ³⁰. And the problem was not one molecule's quirk. A review that year argued that body-temperature maintenance is the predominant function of TRPV1, an evolutionarily conserved role from rodents to primates, and that the pain and temperature roles are therefore intimately linked ³¹.

The reversal of the reversal has taken sixteen years and is not finished. In 2012 a Novartis group described BCTP, a classic polymodal inhibitor that blocked capsaicin activation of the human channel with a half-maximal inhibitory concentration of 65.4 nanomolar and low-pH activation at 26.4 nanomolar, was analgesic in rats at 2 milligrams per kilogram by mouth, and raised core temperature by at most 0.6 degrees Celsius even at 30 and 100 milligrams per kilogram, against more than 1 degree for AMG 517 at doses giving less analgesia ⁴³. In 2024 Huang and colleagues gave the structural reason. Comparing four antagonists in wild-type and mutant mice, they found that the hyperthermia tracked with binding to the linker between transmembrane segments 4 and 5 where it faces the vanilloid pocket, and not with the drug's effect on proton gating, the route of administration, hair coverage, the daily temperature rhythm or inflammation. A compound they identified, PSFL2874, does not bind there, is effective against inflammatory pain, and does not change core body temperature. Their conclusion is that nociception and temperature control are coupled to the channel through distinct, biased allosteric routes ³².

Meanwhile the field moved where the drug does not have to be swallowed. SAF312, generic name libvatrep, is a TRPV1 antagonist formulated as an eye drop. In a 40-participant masked study after bilateral photorefractive keratectomy, in which both eyes were operated and each patient's two eyes were randomized to drug and to vehicle, the eye randomized to SAF312 had pain scores 11.13 points lower on a 100-point visual analog scale at 6 hours, p equals 0.005, a 25 percent difference, and 8.56 points lower averaged over the first 12 hours, p equals 0.017; no serious adverse events occurred and wound healing was not delayed ⁴⁴. The study was designed and co-authored by Novartis. A local antagonist in an organ with its own thermal irrelevance is a sensible way around a systemic thermoregulatory problem, and it is a small trial in a narrow indication, not a revival of the class. The skin route has now been tested too. ACD440 Gel, a topical TRPV1 antagonist, was applied twice daily for seven days in a randomized, double-blind, placebo-controlled crossover phase 2a study of 14 patients with chronic peripheral neuropathic pain and sensory hypersensitivity, from a mixture of postherpetic neuralgia, postoperative nerve injury and chemotherapy. A period effect forced the analysis onto first-period data alone, which makes it a parallel-group comparison of 14 people; in it, the gel reduced the pain evoked by a 40-degree-Celsius thermoroller, p equals 0.029, with no adverse events attributed to the treatment, while evoked mechanical hyperalgesia and brush allodynia were not significantly changed, p equals 0.07 ⁴⁵. The study was sponsored by AlzeCure Pharma, two of whose employees are authors. Fourteen patients in an exploratory study is a signal for the local-antagonist idea, not a proof of it. Reviews of knee osteoarthritis now summarize the state honestly: antagonists can induce analgesia, but hyperthermia and thermal hypesthesia have limited their use, and clinical development of agonists has progressed further ³⁶.

Resiniferatoxin: the molecular scalpel, and what the trials actually show

Resiniferatoxin is a diterpene carried in the latex of Euphorbia resinifera, a cactus-like spurge endemic to northern and central Morocco ¹⁴, and it is the most potent vanilloid agonist known ^6,34. Its usefulness is not that it is strong but that it is selective in a destructive way. Because only TRPV1-expressing neurons admit it, and because the calcium load it produces is prolonged rather than brief, it kills or disables those cells and spares everything else. Michael Iadarola and Andrew Mannes set out the logic in 2011: the compound cannot be given systemically, but it is adaptable to almost any local route, subcutaneous, into a ganglion, or into the cerebrospinal fluid around the spinal cord ⁴⁶.

The mechanism turned out to be more surgical than expected. Studying rats, client-owned dogs and one human subject with advanced cancer, Sapio and colleagues found that intrathecal resiniferatoxin produces strong analgesia with surprisingly little change in gene expression in the dorsal root ganglion, and that it largely spares the sensory cell bodies, which remain active at the periphery but can no longer transmit to the spinal cord. They named the process central chemo-axotomy: a chemical cutting of the central branch of the nerve ²⁴.

The veterinary evidence is the strongest controlled evidence that exists for this drug. Seventy-two companion dogs with painful bone cancer were randomized to standard analgesia alone or standard analgesia plus a single intrathecal dose of 1.2 micrograms per kilogram of resiniferatoxin. Within six weeks, 78 percent of control dogs required unblinding for a change of analgesic protocol or euthanasia, against 50 percent of treated dogs, p less than 0.03, and controls required unblinding significantly sooner, p less than 0.02. No deafferentation pain syndrome, the feared complication of destroying sensory nerves, was seen ³⁴.

The human cancer pain data are early and should be read as such. In May 2025 an interim analysis of a first-in-human, open-label phase 1 study was published, 19 patients with refractory pain from advanced cancer in the abdomen or lower limbs given one intrathecal dose. The primary outcome was safety. Over 188 days there were 213 treatment-emergent adverse events, including 37 serious ones in 14 patients, and nine deaths at an average of 70 days after treatment, range 11 to 140 days, which the authors note are consistent with the course of advanced cancer. Three patients lost heat sensation in the treated dermatomes, grades one and two. Seven had urinary retention beyond 24 hours, three of them grade three. Five had transient prolongation of the electrocardiographic QT interval that resolved within a day. Worst pain fell from 8.4 plus or minus 0.4 to 5.2 plus or minus 0.6, a 38 percent reduction, and opioid consumption fell 57 percent by day 15 ¹⁵. There is no control group, the patients are dying, and the pain numbers are secondary outcomes from an interim look. It is a promising and genuinely novel single-dose, opioid-sparing analgesic, and it is not yet a proven one.

Other routes are further along in some places and nowhere in others. Intravesical resiniferatoxin, instilled into the bladder, restored continence in a subset of patients with idiopathic and neurogenic detrusor overactivity ¹⁶. Intra-articular injection for moderate to severe knee osteoarthritis has been through phase 1; as of a 2025 review, those results had been reported only in abstract form, which the review itself flags as preliminary ⁴⁷. That review was co-authored by an employee of Vivasor, the company developing the compound, and a 2024 review of the same target in knee osteoarthritis was co-authored by an employee of Grünenthal; neither fact makes the biology wrong, and both belong on the page ^36,47.

The intra-articular question did not stay open. A 2026 critical appraisal of the whole TRPV1 program in osteoarthritis records that intra-articular TRPV1 agonists, despite expedited regulatory designations, failed to meet their primary endpoints in phase 3 trials, and that the target has now completed a full translational cycle without yielding a regulatory approval: the mechanistic rationale held, the clinical translation did not. The authors attribute the failure to preclinical models that capture central sensitization poorly, to trials that did not stratify patients by peripheral against central pain, and to composite pain endpoints in mixed populations ⁴⁸. Two of them declare company affiliations, PharmInVivo Ltd. and Organelle Therapeutic Inc., and one is also an author of the 2024 Grünenthal-affiliated review cited above ³⁶.

The newest agonist result did not come from resiniferatoxin at all. Vocacapsaicin is a prodrug that releases capsaicin slowly in the tissue where it is placed. In a triple-blinded, randomized, placebo-controlled, dose-ranging trial, 147 patients having bunion surgery received 14 milliliters of placebo or of one of three concentrations instilled into the surgical site during the operation. At the highest concentration, 0.30 milligrams per milliliter, pain at rest over the first 96 hours, the prespecified primary endpoint, was 33 percent lower than with placebo, interval 10 to 52 percent, p equals 0.005; opioid consumption over the same 96 hours was 50 percent lower, interval 26 to 67 percent, p equals 0.002; and 26 percent of those patients required no opioid at all, against 5 percent on placebo, p equals 0.025. The separation persisted for at least two weeks and no safety measure differed between groups ⁴⁹. It is one trial, in one operation, designed and co-authored by Concentric Analgesics, which owns the drug; but it is a positive randomized trial of a TRPV1 agonist in acute surgical pain, a setting the patch and resiniferatoxin never entered. A 2026 review of the field counts capsaicin formulations, resiniferatoxin, CNTX-4975 and vocacapsaicin among the agonists in development and NEO6860, mavatrep and ACD440 among the antagonists, and describes the clinical developments of the last two years as the first positive ones in some time ⁵⁰.

TRPV1 in the gut, the vagus nerve and the dorsal vagal complex

TRPV1 is not only a skin protein. It is carried by vagal afferents, the sensory fibers that run from the gut up the vagus nerve to the brainstem, with their cell bodies in the nodose ganglia in the neck, and it is present on their central terminals in the dorsal vagal complex, the brainstem region where gut information arrives ⁸. That is the anatomical basis for everything that follows about nausea, vomiting and capsaicin applied to the abdomen.

In the brainstem the channel does something more interesting than carrying pain. Recording from neurons of the dorsal motor nucleus of the vagus in rat brainstem slices, Derbenev and colleagues found that capsaicin rapidly increased inhibitory input onto those neurons, that anandamide mimicked the effect, and that vanilloid receptor antagonists blocked it. The pathway was indirect: TRPV1 on glutamate-releasing terminals increased glutamate release, which in turn drove GABA-releasing interneurons, so the net effect on the vagal motor output was inhibition ⁵¹. An endocannabinoid acting through a capsaicin receptor to inhibit the vagal motor nucleus is a specific, measured mechanism in rat tissue; it is not a demonstration of anything in a human stomach.

The channel also shapes how gut hormones are heard. Cholecystokinin is released from the upper intestine during a meal and promotes the feeling of having eaten enough by exciting vagal afferents. Arnold and colleagues showed in 2023 that at higher concentrations this signal runs through TRPV1: cholecystokinin responses were larger in TRPV1-containing afferents, and blocking the channel's pore or deleting the gene reduced the response in cultured vagal neurons and abolished the hormone's ability to increase spontaneous glutamate release onto neurons of the nucleus of the solitary tract ⁸. Earlier comparative work had already shown that cholecystokinin activation of vagal neurons was attenuated but not abolished by a broad TRP channel blocker, and that capsaicin sensitivity itself varies tenfold between mouse strains, a caution about reading any one rodent result as the biology ⁵².

The newest work finds the arrangement is not even symmetrical. A 2026 study comparing left and right nodose ganglia reported greater expression of TRP channel genes including TRPV1, and more TRPV1-positive afferents, on the left, while the right carried more cholecystokinin receptor and responded more strongly to the hormone ⁵³. All of this is rodent neurophysiology. It establishes that TRPV1 is positioned to modulate the gut-brainstem circuit that produces nausea and vomiting; it does not establish that rubbing capsaicin on a human abdomen reaches that circuit.

Cannabinoid hyperemesis syndrome: the hot shower, and the cream

In 2004 J. H. Allen and colleagues in South Australia described nineteen patients with heavy chronic cannabis use and a cyclical vomiting illness. Five refused consent and five were excluded, leaving nine cases reported with one previously published case. In every one, chronic cannabis use preceded the vomiting. Stopping cannabis ended the illness in seven; three who did not stop kept vomiting; three who stopped and then deliberately resumed became ill again, and two of those stopped a second time and recovered. The finding the authors called novel was that nine of the ten displayed an abnormal washing behavior during attacks ⁵⁴. That is the whole origin of the syndrome, and it is worth noticing how small it is: nine patients, and a pattern that has since been confirmed thousands of times.

The washing is the diagnostic signature. A 2017 systematic review screened 1,253 abstracts and included 183 articles. Across the pooled cases, a history of regular cannabis use was present in 100 percent, cyclic nausea and vomiting in 100 percent, resolution after stopping cannabis in 96.8 percent, compulsive hot bathing with symptom relief in 92.3 percent, abdominal pain in 85.1 percent, at least weekly use in 97.4 percent, and male predominance in 72.9 percent. The same review stated flatly that the underlying pathophysiology remains unclear, with a dearth of research into it ¹⁷. Nothing published since has changed that sentence.

The TRPV1 hypothesis is an inference from the shower, and it should be labeled as one. The reasoning runs: very hot water activates TRPV1 in the skin; sustained activation defunctionalizes those fibers; TRPV1 is also present on the gut's vagal afferents and in the dorsal vagal complex; chronic cannabinoid exposure may dysregulate a system in which anandamide is itself a TRPV1 agonist; therefore capsaicin, which activates the same channel more directly, should reproduce the relief of the shower ²¹. Every link in that chain is plausible and most are supported in animals ^5,8,51. None of it has been demonstrated in a person with the syndrome. The 2017 systematic review, a 2020 pharmacy review and a 2026 case report all describe the mechanism as proposed rather than established: the 2020 review puts it as relief produced perhaps through antiemetic effects of TRPV1 activation ^17,21,55.

The clinical evidence, at its true size. The first signals were case reports: a 41-year-old woman whose intractable vomiting resolved within 24 hours of topical capsaicin over the epigastrium after fluids, antiemetics and morphine had failed ⁵⁶; and in 2026, a 28-year-old man using about 2 grams of high-potency cannabis daily for seven years in whom about 2 grams of 0.1 percent capsaicin cream across the abdomen stopped retching within 10 minutes and brought pain from 8 out of 10 to 2 by 30 minutes ⁵⁵. Then a retrospective cohort of 43 emergency department patients: length of stay was not significantly shortened, 201 against 179 minutes, p equals 0.33, but patients given capsaicin received fewer additional medications, a median of 4 against 3 doses, p equals 0.015, 67 percent needed nothing further before discharge, and opioid use was lower, 166.5 against 69 morphine milligram equivalents ⁵⁷.

Then the one randomized trial of capsaicin that exists. Dean and colleagues ran a double-blind, placebo-controlled pilot in 30 emergency department adults, 17 given 0.1 percent capsaicin cream to the anterior abdomen and 13 placebo. The primary endpoint, nausea on a 0 to 10 centimeter visual analog scale at 30 minutes, was 4.1 plus or minus 2.3 against 6.1 plus or minus 3.3, a difference of 2.0 centimeters whose 95 percent interval, reported as 0.2 to minus 4.2, includes no difference; the trial missed its primary endpoint. At 60 minutes the difference was larger, 3.2 plus or minus 3.2 against 6.4 plus or minus 2.8, interval minus 0.9 to minus 5.4, and complete resolution of nausea occurred in 29.4 percent of the capsaicin group against none of the placebo group, a relative risk of 3.4, interval 1.6 to 7.1. One patient could not tolerate the cream because of skin irritation ¹⁹. Thirty patients, one center, a missed primary endpoint and a positive secondary one: that is the entire randomized evidence base for a treatment now in wide use.

The comparison drug has better evidence. The HaVOC trial randomized cannabis users with active vomiting to intravenous haloperidol, a dopamine antagonist, or ondansetron, in a triple-blind design; 33 were enrolled and 30 received treatment, 13 haloperidol and 17 ondansetron, with a mean age of 29 and reported use of 1.5 grams a day since age 19. Haloperidol beat ondansetron on the combined reduction in abdominal pain and nausea by 2.3 centimeters, interval 0.6 to 4.0, p equals 0.01, with less rescue medication, 31 against 59 percent, and a shorter time to departure, 3.1 against 5.6 hours, p equals 0.03. Two patients returned with acute dystonia, both from the higher haloperidol dose ⁵⁸. That the dopamine blocker works better than the serotonin blocker is itself a clue about mechanism, and it is a clue that does not point at TRPV1.

The syntheses agree with each other about how weak all of this is. A 2017 review found 63 articles and 205 subjects, of which only four were level-2 prospective studies with 64 subjects between them, and concluded that high-quality evidence for pharmacologic treatment is limited, with benzodiazepines, then haloperidol and capsaicin, most often reported effective acutely, and hot showers cited as universally effective in every case series and case report ⁵⁹. A 2024 systematic review for the Society for Academic Emergency Medicine found seven studies and 492 patients in total, five of capsaicin with 386 patients and two of dopamine antagonists with 106, and reported mixed evidence for capsaicin and benefit for the dopamine antagonists ²⁰. The resulting guideline suggests both haloperidol or droperidol and topical capsaicin, each in addition to usual care, and labels the certainty of evidence for both as very low ¹⁸. A 2025 systematic review found 34 eligible articles with 63 individual cases, again reporting capsaicin cream, antipsychotics and benzodiazepines as more effective than promethazine, ondansetron or metoclopramide, while noting that its own search ran only to July 2021 and that the algorithm it proposes rests on case reports ⁶⁰.

The condition is growing, which is why this matters. In Alberta, a province of about five million, emergency department visits for vomiting with a concurrent cannabis diagnostic code rose from 15 per 100,000 population, interval 14 to 17, in 2017 to 2018, before recreational legalization, to 21, interval 20 to 23, after legalization, to 32, interval 31 to 35, in the first year of the COVID-19 pandemic, p less than 0.001; among chronic cannabis users aged 16 to 24 the rate reached 6 per 1,000. In the accompanying survey of 157 sufferers, problematic cannabis use was universal, and 59 and 68 percent screened positive for at least moderate anxiety and depression ⁶¹. A 2025 internet survey of 1,052 people with self-reported cannabinoid hyperemesis syndrome found frequent use, heavy health care utilization, and an association with use starting in adolescence; it is a self-selected online cohort and cannot give a population prevalence ⁶². One 2026 clinical review states that the syndrome affects approximately one-third of cannabis users; no population data support a figure of that size, and it should not be repeated ⁶³.

The pediatric record has grown in the past year. In 2025, two adolescents at one hospital who met the Rome IV criteria for the syndrome and had not responded to the usual antiemetics, capsaicin among them, over 36 to 45 hours were given aprepitant, a blocker of the neurokinin-1 receptor, which is the receptor for substance P; vomiting stopped within an hour, fluids were tolerated within one to two hours, solid food within four to eight, and both were discharged the next day ⁶⁴. That is two patients, unblinded, with no control group, and it is mechanistically interesting mainly because so much of the older capsaicin story was told in terms of substance P. In 2026, a pediatric emergency department reported what standardizing its approach actually achieved: across 128 encounters by 44 adolescents between 2020 and 2024, capsaicin administration rose from 2.7 to 22.2 percent and metoclopramide from 6.8 to 42.6 percent, both p less than 0.001, and the mean haloperidol dose fell from 2.7 to 1.0 milligrams, p equals 0.014, while the adjusted odds of admission, 0.57, interval 0.17 to 1.86, and the length of stay were unchanged ⁶⁵. Changing what is given is not the same as changing what happens to the patient.

Two practical cautions. In adolescents, haloperidol carries a real risk. Two cases at one children's hospital: a 15-year-old with hypokalemia and hypomagnesemia whose corrected QT interval reached 528 milliseconds on a haloperidol infusion, which was then stopped, and a 17-year-old with a borderline QT interval of 476 milliseconds whose nausea and vomiting were treated instead with intravenous fosaprepitant. The same report's literature search through October 2024 found five severe cases of QT prolongation or torsades de pointes in adolescents and young adults, which is one of the reasons topical capsaicin is proposed as an alternative in that age group ⁶⁶. A single-center series of six adolescents reported full relief in four given haloperidol with lorazepam and in one given haloperidol with peri-umbilical 0.025 percent capsaicin, while the one patient treated with topical capsaicin alone improved but had persistent nausea ⁶⁷. And the only intervention with consistent evidence remains the one nobody can bill for: stopping cannabis ^17,54.

TRPV1 and migraine: a good mechanism and a thin clinical record

The mechanistic case is straightforward. Migraine pain involves release of calcitonin gene-related peptide from trigeminal sensory nerve endings onto the meninges and their vessels, and TRPV1 is one of the channels whose opening releases it. Working with rat preparations of the hemiskull with attached dura, the trigeminal ganglion and the trigeminal nucleus caudalis, Citak and colleagues showed that agonists of TRPV1, along with TRPV4, TRPA1 and TRPM8, each evoked calcitonin gene-related peptide release, and that each release was blocked by the matching selective antagonist ⁶⁸. That is clean in vitro pharmacology in rat tissue. It is not evidence that blocking TRPV1 helps a person with migraine.

The genetics are suggestive and weak. A Spanish case-control study genotyped 149 variants across 14 TRP genes in 1,040 migraine cases and 1,037 controls in two stages, a discovery sample of 555 and a replication sample of 485. Nineteen variants reached nominal significance in the discovery stage; after replication, only two survived nominally, one in TRPV1 and one in TRPV3, and none of the risk haplotypes replicated ⁶⁹. A Chinese study of 251 migraine patients and 600 controls genotyped 13 variants in nine genes and found two associated with increased migraine risk in a dominant model, rs8065080 with an odds ratio of 1.75, interval 1.09 to 2.90, p equals 0.025, and rs7217270 with 1.63, interval 1.02 to 2.58, p equals 0.039 ⁷⁰. These are small candidate-gene studies with nominal p values and no genome-wide correction. They are consistent with a role; they establish nothing.

The one headache trial of a TRPV1 drug is old, small and about cluster headache rather than migraine. Civamide, the cis isomer of capsaicin, was given as a nasal solution, 25 micrograms per nostril once daily for seven days, to 28 patients with episodic cluster headache randomized two to one, 18 to drug and 10 to vehicle. The civamide group had a significantly greater percentage decrease in headaches during treatment days 1 to 7, minus 55.5 against minus 25.9 percent, p equals 0.03, with trends in the same direction over the following weeks that did not reach significance. Nasal burning occurred in 14 of 18 on drug against 1 of 10 on vehicle, p equals 0.001, which makes the blinding questionable ⁷¹. Twenty-eight patients, in 2002, in a different headache disorder, with an adverse effect that announces the allocation to the patient. The honest summary is that TRPV1 is a credible participant in trigeminal pain and has never been shown to be a useful drug target in headache.

Capsaicin and cancer: both claims, at their true strength

Two opposite claims circulate about capsaicin and cancer, and both have real papers behind them. The useful thing is to notice what kind of study each rests on.

The claim that it causes cancer comes mostly from gastric cancer case-control studies in populations that eat a great deal of chili. A population-based study in Mexico City in 1989 to 1990 compared 220 incident gastric cancer cases with 752 randomly selected population controls. Chili pepper consumers had an age- and sex-adjusted odds ratio of 5.49 for gastric cancer, interval 2.72 to 11.06, and the odds ratio for self-rated high-level consumers against non-consumers was 17.11, interval 7.78 to 37.59. The authors flagged their own problem: when consumption was measured as frequency per day rather than self-rated level, the significant trend among consumers disappeared, and they wrote that further studies were needed to test the hypothesis ⁷². A far smaller Mexican study in 2018, 46 cases against 46 blood-bank controls, found consumption above 29.9 milligrams of capsaicin a day associated with gastric cancer in a multivariable model, odds ratio 3.77, interval 1.21 to 13.11, alongside lack of formal education at 17.47 and an unsuitable breakfast at 8.99, which gives a sense of how much confounding by social circumstance such a model is carrying ⁷³. Case-control studies of diet recalled after a cancer diagnosis are among the weakest designs in epidemiology, and an odds ratio of 17 with a wide interval in a self-rated exposure should raise suspicion of recall bias rather than confidence in a dose-response.

The claim that it prevents cancer, or at least death, comes from prospective cohorts, which are a stronger design, and the finding there is about mortality rather than incidence. A meta-analysis of four cohorts with 564,748 participants, 31,527 deaths from all causes, 10,184 from cardiovascular disease and 9,868 from cancer, found that chili consumption against none or rare was associated with lower all-cause mortality, hazard ratio 0.87, interval 0.85 to 0.90, lower cardiovascular mortality, 0.89, interval 0.85 to 0.93, and lower cancer-specific mortality, 0.92, interval 0.88 to 0.97 ⁷⁴. An Italian cohort reported separately, Moli-sani, with 22,811 adults followed a median of 8.2 years and 1,236 deaths, found eating chili more than four times a week against none or rarely associated with a hazard ratio of 0.77 for all-cause death, interval 0.66 to 0.90, and 0.66 for cardiovascular death, interval 0.50 to 0.86 ⁷⁵. The Golestan cohort in Iran, 44,398 adults analyzed over 11 years with 5,121 deaths, found black or chili pepper consumption associated with a hazard ratio of 0.91 for overall mortality, interval 0.86 to 0.98, and found no association between any studied spice and cancer mortality ⁷⁶. These are observational associations in people who differ in many other ways from those who do not eat chili, and none of them tests capsaicin as such.

The laboratory work is where the apoptosis headlines come from, and it is cell culture. Capsaicin lowered the half-maximal inhibitory concentration of fluorouracil several-fold in the HGC-27 gastric cancer cell line and was associated with cytochrome c release, a marker of the mitochondrial route to programmed cell death; the authors themselves open by calling the role of capsaicin in carcinogenesis quite controversial and close by calling for well-controlled epidemiologic studies ⁷⁷. A separate group found that dihydrocapsaicin, not capsaicin, was the chili component with the strongest inhibition of growth-factor-induced neoplastic transformation in cultured cells, acting through the p70S6K1 and c-Fos pathways ⁷⁸. Neither study involved an animal with a tumor, let alone a person. Neither of the PubMed records for these two papers carries a retraction notice or expression of concern as of 19 October 2026.

What can be said: in human cells in a dish, capsaicin and its relatives can trigger cell death and can sensitize cancer cells to chemotherapy. In populations, eating chili is associated with slightly lower overall and cardiovascular mortality and, in one meta-analysis, slightly lower cancer mortality; in two Mexican case-control studies it is associated with gastric cancer. No randomized trial of capsaicin for the prevention or treatment of any cancer exists. Nothing on either side of this is strong enough to change what anybody eats.

What changed, and what is still unknown

Five things that were believed are not. TRPV1 is not the heat sensor; it is one of three partly redundant ones, and acute heat withdrawal survives the loss of any one or two of them ²³. The analgesia of topical capsaicin is not caused by the depletion of substance P; that is a correlate, and the cause is defunctionalization of the nerve terminal itself ¹². The hyperthermia of TRPV1 antagonists is not an unavoidable consequence of blocking the channel; it tracks with binding at one specific region facing the vanilloid pocket, and a compound avoiding that region was analgesic in mice without raising temperature ³². And the conclusion of 2008, that selective antagonists cannot be developed, was correct about every drug then in the clinic and is now, carefully, under revision ^10,32,44. And the expectation that a TRPV1 agonist injected into an arthritic knee would relieve its pain did not survive phase 3: intra-articular agonists missed their primary endpoints despite expedited regulatory designations ⁴⁸.

Several things remain genuinely open. Nobody knows whether anandamide reaches TRPV1 at relevant concentrations in an intact human, or under what circumstances; the endovanilloid concept rests on isolated vessels, transfected cells and pharmacological tools that themselves act on the channel ^5,28. Nobody has shown that capsaicin applied to human abdominal skin alters activity anywhere in the dorsal vagal complex; the circuit exists in rats and the inference to people is untested ^8,51,53.

The pathophysiology of cannabinoid hyperemesis syndrome is unknown, and the 2017 systematic review's phrase, a dearth of research dedicated to investigating its underlying mechanism, still applies ¹⁷. The TRPV1 explanation of the hot shower is the best story available and has never been tested in a patient. Why a dopamine antagonist outperforms a serotonin antagonist, and why the syndrome affects a minority of heavy users and not the rest, are both unexplained ^58,61.

The clinical questions that could be answered and have not been: whether topical capsaicin beats placebo for cannabinoid hyperemesis syndrome in a trial larger than 30 patients ^19,20; whether intrathecal resiniferatoxin improves anything in a controlled comparison rather than an open-label phase 1 ¹⁵; whether any intra-articular agonist helps knee osteoarthritis, now that the phase 3 trials have missed their primary endpoints and the resiniferatoxin phase 1 data exist only as conference abstracts ^47,48; and whether a hyperthermia-sparing antagonist does in a person what PSFL2874 did in a mouse ³².

Two structural questions sit underneath all of this. How heat opens the channel at the level of atoms is still not settled, even with high-resolution structures, because temperature is not a ligand that can be seen bound in a map ^3,27. Cryo-electron microscopy has now caught temperature-induced open states of several heat-sensitive TRP channels, but a 2026 review of those six studies concludes that how temperature itself drives the opening is still poorly understood ⁷⁹. And the field has not explained how one pore integrates heat, protons, lipids and an exogenous plant vanillylamide into a single output without any of them interfering with the others, which is what the dual-gate, allosterically coupled model was proposed to address and what the biased-agonism finding of 2024 shows is only partly understood ^27,32,35.

A note on who is paying. Nearly every clinical dataset on this page was generated by a company with a product: Amgen for the antagonists ^10,29,30,31, Novartis for BCTP and libvatrep ^43,44, NeurogesX and later Astellas and Grünenthal for the capsaicin patch ^11,38,39,41, Vivasor for resiniferatoxin in osteoarthritis ⁴⁷, Grünenthal again for the 2024 review of the target in knee osteoarthritis ³⁶, Concentric Analgesics for vocacapsaicin ⁴⁹ and AlzeCure Pharma for ACD440 ⁴⁵. The National Institutes of Health intramural program funded the intrathecal resiniferatoxin work, which is one of the few lines here without a commercial sponsor ^15,24. The declared interests do not make any of these findings false, and several of the most damaging results, including the hyperthermia that killed the antagonist class, were published by the sponsoring company itself ¹⁰. They do mean that absence of an unfavorable result is not evidence that none was found. The cheapest intervention on this page, stopping cannabis, has no sponsor and the most consistent evidence ^17,54; the second cheapest, a tube of over-the-counter capsaicin cream, has one small pilot trial and a very-low-certainty guideline suggestion behind it ^18,19.

The tests, one by one

• Skin biopsy for intraepidermal nerve fiber densityA 3-millimeter punch of skin is stained for the nerve marker PGP 9.5 and the small fibers crossing into the epidermis are counted. It is the only way to see what a TRPV1 agonist has done to a person's nerve endings: one 60-minute application of the 8 percent patch cut density by about 80 percent at one week, with regeneration evident but incomplete at 12 weeks and about 93 percent recovery at 24 weeks ¹¹.To document small-fiber neuropathy, and in trials to show that capsaicin defunctionalizes rather than destroysQuantitative and reproducible enough to be the pharmacodynamic measure in the patch developer's own randomized, controlled, open-label study of long-term effects in healthy volunteers ¹¹It counts fibers, not channels, and says nothing about whether the remaining fibers carry TRPV1 or whether they work; it also cannot be done on the organ that matters in gut symptoms ⁸
• Quantitative sensory testing, including a capsaicin or heat challengeThresholds for warmth, cold, touch and sharp mechanical pain are measured against calibrated stimuli, sometimes after a deliberate capsaicin application to create a controlled area of hypersensitivity. After the 8 percent patch, tactile thresholds rose about 8 percent and sharp mechanical pain reports fell about 15 percentage points, while heat- and cold-detection thresholds did not change significantly ¹¹. In the 52-week diabetic neuropathy study, testing of sharp, warm, cold and vibration stimuli showed no worsening with repeat treatment ³⁹.To check that a defunctionalizing treatment has not cost the patient protective sensationGood for group comparisons in a trial; thresholds vary widely between individuals and with attention, so a single patient's value is hard to interpretBecause TRPV1 is only one of three redundant heat sensors, heat thresholds can stay normal while TRPV1-bearing fibers are gone ^11,23
• Genotyping of TRPV1 variantsSequencing or array genotyping of the gene at 17p13 ². This is a research tool with no clinical role. The migraine association studies are the clearest illustration of its limits: 149 variants in 1,040 cases and 1,037 controls left only two nominally replicated and no replicated haplotype ⁶⁹, and a 251-patient Chinese study found odds ratios of 1.75 and 1.63 at uncorrected nominal significance ⁷⁰.Research into pain and headache susceptibility onlyThe genotype call is accurate; the inference from genotype to risk is notEverything that matters clinically. No test exists that measures how much TRPV1 a given person has, or how sensitized it is

Where it connects

Sources

79 sources, numbered as they are cited. Every one was checked against PubMed or its publisher before it was cited here; the note under each says what it shows and what it does not.

1. 1
   Caterina MJ, Schumacher MA, Tominaga M, Rosen TA, Levine JD, Julius D. The capsaicin receptor: a heat-activated ion channel in the pain pathway.Nature · 1997 · 389(6653):816-24doi:10.1038/39807 · PMID 9349813

   The cloning paper. Expression cloning based on calcium influx isolated a complementary DNA from sensory neurons encoding a non-selective cation channel structurally related to the TRP family, also activated by increases in temperature in the noxious range. It establishes the molecule and its two main stimuli; it does not establish what the channel does in an intact animal.

2. 2
   Hayes P, Meadows HJ, Gunthorpe MJ, Harries MH, Duckworth MD, Cairns W, et al. Cloning and functional expression of a human orthologue of rat vanilloid receptor-1.Pain · 2000 · 88(2):205-215doi:10.1016/S0304-3959(00)00353-5 · PMID 11050376

   Source for the human gene: a 2,517-base-pair open reading frame, 92 percent homology to rat, chromosomal location 17p13, expression in human dorsal root ganglia and at low levels across a wide range of central and peripheral tissues, with responses to capsaicin, pH and temperature.

3. 3
   Liao M, Cao E, Julius D, Cheng Y. Structure of the TRPV1 ion channel determined by electron cryo-microscopy.Nature · 2013 · 504(7478):107-12doi:10.1038/nature12822 · PMID 24305160

   The 3.4-angstrom structure: four-fold symmetry around a central pathway formed by transmembrane segments 5 and 6 and the pore loop, flanked by segment 1 to 4 voltage-sensor-like domains, a wide extracellular mouth, a short selectivity filter, the TRP domain against the segment 4 to 5 linker, and N-terminal ankyrin repeats. Rat TRPV1 expressed in HEK293 cells.

4. 4
   Caterina MJ, Leffler A, Malmberg AB, Martin WJ, Trafton J, Petersen-Zeitz KR, Koltzenburg M, Basbaum AI, Julius D. Impaired nociception and pain sensation in mice lacking the capsaicin receptor.Science · 2000 · 288(5464):306-13doi:10.1126/science.288.5464.306 · PMID 10764638

   VR1-null mice show no vanilloid-evoked pain behavior, impaired detection of painful heat, little thermal hypersensitivity during inflammation, and normal responses to noxious mechanical stimuli. Also the source of the quoted activation threshold above 43 degrees Celsius. Mouse genetics; impairment, not abolition, of heat sensing.

5. 5
   Zygmunt PM, Petersson J, Andersson DA, Chuang H, Sørgård M, Di Marzo V, Julius D, Högestätt ED. Vanilloid receptors on sensory nerves mediate the vasodilator action of anandamide.Nature · 1999 · 400(6743):452-7doi:10.1038/22761 · PMID 10440374

   The founding endovanilloid paper. Anandamide-induced vasodilation in isolated arteries was capsaicin-sensitive, released calcitonin gene-related peptide, was blocked by capsazepine but not by a CB1 antagonist, and other cannabinoid agonists did not mimic it; anandamide induced capsazepine-sensitive currents in cells expressing the cloned receptor. Isolated guinea pig and rat tissue, not intact humans.

6. 6
   Szallasi A, Blumberg PM. Vanilloid receptors: new insights enhance potential as a therapeutic target.Pain · 1996 · 68(2-3):195-208doi:10.1016/s0304-3959(96)03202-2 · PMID 9121806

   Pre-cloning review describing resiniferatoxin as the ultrapotent analog of capsaicin, the restriction of vanilloid binding to primary sensory neurons involved in nociception and neurogenic inflammation, desensitization as the therapeutic strategy, and irritancy as the limiting problem. Useful for the history, not for any modern number.

7. 7
   Tominaga M, Caterina MJ, Malmberg AB, Rosen TA, Gilbert H, Skinner K, Raumann BE, Basbaum AI, Julius D. The cloned capsaicin receptor integrates multiple pain-producing stimuli.Neuron · 1998 · 21(3):531-43doi:10.1016/s0896-6273(00)80564-4 · PMID 9768840

   Heat-evoked single-channel currents in excised membrane patches show heat gates the channel directly; protons lower the temperature threshold so that pH 5.9 or below activates it at room temperature; the receptor is located in a neurochemically heterogeneous population of small-diameter primary afferents. Rat and cloned-channel work.

8. 8
   Arnold RA, Fowler DK, Peters JH. TRPV1 enhances cholecystokinin signaling in primary vagal afferent neurons and mediates the central effects on spontaneous glutamate release in the NTS.Am J Physiol Cell Physiol · 2023 · 326(1):C112-C124doi:10.1152/ajpcell.00409.2023 · PMID 38047304

   Calcium imaging and brainstem electrophysiology showing that cholecystokinin signaling at high concentrations is potentiated in TRPV1-containing vagal afferents, and that pore block or genetic deletion of TRPV1 reduces the response and eliminates the hormone's ability to increase spontaneous glutamate release in the nucleus of the solitary tract. Rodent.

9. 9
   Logan DW. Hot to touch: the story of the 2021 Nobel Prize in Physiology or Medicine.Dis Model Mech · 2021 · 14(10)doi:10.1242/dmm.049352 · PMID 34704598

   Editorial recording that the 2021 prize went to Ardem Patapoutian and David Julius for receptor channels underlying touch and temperature, and that somatosensation was the fourth sensory modality recognized by the committee. A source for the award, not for any experimental result.

10. 10
    Gavva NR, Treanor JJS, Garami A, Fang L, Surapaneni S, Akrami A, et al. Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans.Pain · 2008 · 136(1-2):202-10doi:10.1016/j.pain.2008.01.024 · PMID 18337008

    The decisive human result. Phase 1 trials of AMG 517 produced marked, reversible, generally plasma-concentration-dependent hyperthermia; after molar extraction it was long-lasting with maximal body temperature surpassing 40 degrees Celsius. In rats the drug caused tail skin vasoconstriction and increased thermogenesis. The authors conclude such antagonists cannot be developed for systemic use as stand-alone agents. An Amgen paper reporting its own program's failure.

11. 11
    Kennedy WR, Vanhove GF, Lu SP, Tobias J, Bley KR, Walk D, Wendelschafer-Crabb G, Simone DA, Selim MM. A randomized, controlled, open-label study of the long-term effects of NGX-4010, a high-concentration capsaicin patch, on epidermal nerve fiber density and sensory function in healthy volunteers.J Pain · 2010 · 11(6):579-87doi:10.1016/j.jpain.2009.09.019 · PMID 20400377

    The human measurement of defunctionalization: about 80 percent reduction in epidermal nerve fiber density one week after a single 60-minute application, about 8 percent increase in tactile thresholds, a roughly 15 percentage point fall in stimuli reported as sharp mechanical pain, incomplete regeneration at 12 weeks, about 93 percent recovery at 24 weeks, and no significant change in heat- or cold-detection thresholds. Healthy volunteers, sponsored by the patch's developer.

12. 12
    Anand P, Bley K. Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch.Br J Anaesth · 2011 · 107(4):490-502doi:10.1093/bja/aer260 · PMID 21852280

    The source for defunctionalization as the mechanism, and for the explicit correction that depletion of substance P is a correlate with little if any causative role. Also records the regulatory status of the 8 percent patch in the European Union and United States and the 12-week duration after a single 60-minute application. A review, not primary data.

13. 13
    Qureshi Z, Ali MN, Khalid M. An Insight into Potential Pharmacotherapeutic Agents for Painful Diabetic Neuropathy.J Diabetes Res · 2022 · 2022:9989272doi:10.1155/2022/9989272 · PMID 35127954

    Cited for the regulatory fact that in the United States the agents approved for painful diabetic neuropathy are duloxetine, pregabalin, extended-release tapentadol and capsaicin, the last restricted to the feet. A narrative review, used here only for that statement of approval scope.

14. 14
    Boutoub O, Jadhav S, Zheng X, El Ghadraoui L, Al Babili S, Fernie AR, Figueiredo AC, Miguel MG, Borghi M. Biochemical characterization of Euphorbia resinifera floral cyathia.J Plant Physiol · 2024 · 293:154184doi:10.1016/j.jplph.2024.154184 · PMID 38295538

    Cited only for the botany: Euphorbia resinifera O. Berg is endemic to the northern and central regions of Morocco and secretes a poisonous latex containing resiniferatoxin. A plant-physiology paper about the flowers, used here for the plant's identity and range and for nothing pharmacological.

15. 15
    Mannes AJ, Heiss JD, Berger A, Alewine CC, Butman JA, Hughes MS, et al. Treatment of Intractable Cancer Pain with Resiniferatoxin - An Interim Study.NEJM Evid · 2025 · 4(6):EVIDoa2400423doi:10.1056/EVIDoa2400423 · PMID 40423401

    Interim analysis of a first-in-human, open-label phase 1 study, 19 patients with refractory cancer pain of the abdomen or lower limbs given one intrathecal dose. Over 188 days: 213 treatment-emergent adverse events, 37 serious in 14 patients, nine deaths at an average of 70 days, range 11 to 140; three with loss of heat sensitivity, seven with urinary retention beyond 24 hours, five with transient QT prolongation. Worst pain fell from 8.4 to 5.2, 38 percent, and opioid use by 57 percent at day 15. No control group; efficacy is a secondary outcome at an interim look. NCT00804154, funded by the NIH intramural program and others.

16. 16
    Szallasi A. Resiniferatoxin: Nature's Precision Medicine to Silence TRPV1-Positive Afferents.Int J Mol Sci · 2023 · 24(20):15042doi:10.3390/ijms242015042 · PMID 37894723

    Review of the whole resiniferatoxin program: the plant source, the broad therapeutic window, intravesical use restoring continence in a subset of patients with idiopathic and neurogenic detrusor overactivity, the molecular-scalpel framing for cancer pain, and intra-articular trials in knee osteoarthritis then underway. A narrative review by a long-standing advocate of the compound.

17. 17
    Sorensen CJ, DeSanto K, Borgelt L, Phillips KT, Monte AA. Cannabinoid Hyperemesis Syndrome: Diagnosis, Pathophysiology, and Treatment-a Systematic Review.J Med Toxicol · 2017 · 13(1):71-87doi:10.1007/s13181-016-0595-z · PMID 28000146

    The frequency figures used here: regular cannabis use 100 percent, cyclic nausea and vomiting 100 percent, resolution after stopping 96.8 percent, compulsive hot baths with relief 92.3 percent, abdominal pain 85.1 percent, at least weekly use 97.4 percent, male predominance 72.9 percent, from 183 included articles out of 1,253 abstracts. Also the explicit statement that the pathophysiology remains unclear with a dearth of research into it. A synthesis of case material, so the percentages describe reported cases, not a population.

18. 18
    Borgundvaag B, Bellolio F, Miles I, Schwarz ES, Sharif S, Su MK, et al. Guidelines for Reasonable and Appropriate Care in the Emergency Department (GRACE-4): Alcohol use disorder and cannabinoid hyperemesis syndrome management in the emergency department.Acad Emerg Med · 2024 · 31(5):425-455doi:10.1111/acem.14911 · PMID 38747203

    The current guideline position, graded by GRADE. For adults with cannabinoid hyperemesis syndrome the panel suggests haloperidol or droperidol in addition to usual care, very low certainty of evidence, and suggests offering topical capsaicin in addition to usual care, also very low certainty. The certainty label is the important part of the citation.

19. 19
    Dean DJ, Sabagha N, Rose K, Weiss A, France J, Asmar T, et al. A Pilot Trial of Topical Capsaicin Cream for Treatment of Cannabinoid Hyperemesis Syndrome.Acad Emerg Med · 2020 · 27(11):1166-1172doi:10.1111/acem.14062 · PMID 32569429

    The only randomized, double-blind, placebo-controlled trial of capsaicin for this condition. Thirty patients, 17 capsaicin and 13 placebo. Nausea at the 30-minute primary endpoint was 4.1 plus or minus 2.3 against 6.1 plus or minus 3.3 centimeters, a difference of 2.0 with an interval reported as 0.2 to minus 4.2, so the primary endpoint was not met; at 60 minutes 3.2 against 6.4, interval minus 0.9 to minus 5.4; complete resolution 29.4 against 0 percent, relative risk 3.4, interval 1.6 to 7.1. One patient could not tolerate the cream. A pilot, explicitly exploratory.

20. 20
    Sabbineni M, Scott W, Punia K, Manuja K, Singh A, Campbell K, MacKillop J, Balodis I. SAEM GRACE: Dopamine antagonists and topical capsaicin for cannabis hyperemesis syndrome in the emergency department: A systematic review of direct evidence.Acad Emerg Med · 2024 · 31(5):493-503doi:10.1111/acem.14770 · PMID 37391387

    From 53 potentially relevant articles, seven were included: five observational studies and two randomized trials, 492 patients in total, of which five studies with 386 patients examined capsaicin and two with 106 examined dopamine antagonists. Mixed evidence for capsaicin, benefit for the dopamine antagonists, and an explicit call for methodologically rigorous trials of both.

21. 21
    Stumpf JL, Williams LD. Management of Cannabinoid Hyperemesis Syndrome: Focus on Capsaicin.J Pharm Pract · 2020 · 34(5):786-793doi:10.1177/0897190020934289 · PMID 32613883

    The clearest statement of the TRPV1 hypothesis for why hot bathing and topical capsaicin both work: an antiemetic effect produced by activation of TRPV1, similar to that of hot water bathing. A narrative review proposing a mechanism, not testing one.

22. 22
    Davis JB, Gray J, Gunthorpe MJ, Hatcher JP, Davey PT, Overend P, et al. Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia.Nature · 2000 · 405(6783):183-7doi:10.1038/35012076 · PMID 10821274

    An independently generated knockout line. The mice appeared normal across a wide range of behavioral tests including acute noxious thermal stimuli, but carrageenan-induced thermal hyperalgesia was completely absent; the authors conclude alternative mechanisms are sufficient for normal sensation of noxious heat. The first clear published correction of the heat-sensor claim.

23. 23
    Vandewauw I, De Clercq K, Mulier M, Held K, Pinto S, Van Ranst N, et al. A TRP channel trio mediates acute noxious heat sensing.Nature · 2018 · 555(7698):662-666doi:10.1038/nature26137 · PMID 29539642

    The completion of the heat-sensor correction. Heat responsiveness in isolated sensory neurons and in C and A-delta fibers survives if any one of TRPV1, TRPM3 or TRPA1 is functional; only the triple knockout lacks acute withdrawal from noxious heat, while retaining normal cold and mechanical responses and preference for moderate temperatures. Mouse genetics.

24. 24
    Sapio MR, Neubert JK, LaPaglia DM, Maric D, Keller JM, Raithel SJ, et al. Pain control through selective chemo-axotomy of centrally projecting TRPV1+ sensory neurons.J Clin Invest · 2018 · 128(4):1657-1670doi:10.1172/JCI94331 · PMID 29408808

    Rats, client-owned dogs and one human subject with advanced cancer. Strong analgesia with limited transcriptional change in the dorsal root ganglion; intrathecal resiniferatoxin largely spares the cell bodies, which remain peripherally active but can no longer signal to the spinal cord. The mechanistic source for the term central chemo-axotomy. One human subject only.

25. 25
    Jordt SE, Tominaga M, Julius D. Acid potentiation of the capsaicin receptor determined by a key extracellular site.Proc Natl Acad Sci U S A · 2000 · 97(14):8134-9doi:10.1073/pnas.100129497 · PMID 10859346

    Glutamate 600, in the linker between the fifth transmembrane domain and the pore region, sets proton potentiation; glutamate 648 is required for proton-evoked activation without diminishing responses to other stimuli. Site-directed mutagenesis in a heterologous system; the clinical relevance to tissue acidosis is inferred.

26. 26
    Szallasi A, Blumberg PM. Vanilloid (capsaicin) receptors and mechanisms.Pharmacol Rev · 1999 · 51(2):159-212PMID 10353985

    The standard pre-structure reference work on the receptor, published two years after the cloning. The PubMed record carries no abstract, so what it fixes here is the date, authorship and scope of the field's reference review; no DOI is listed in the record.

27. 27
    Cao E, Liao M, Cheng Y, Julius D. TRPV1 structures in distinct conformations reveal activation mechanisms.Nature · 2013 · 504(7478):113-8doi:10.1038/nature12823 · PMID 24305161

    Two activated states obtained with a peptide toxin and small vanilloid agonists. The segment 1 to 4 domain stays stationary, unlike in voltage-gated channels; opening involves rearrangement of the outer pore and dilation of a hydrophobic constriction at the lower gate, a dual gating mechanism with allosteric coupling between the two. Structural, not functional in vivo.

28. 28
    De Petrocellis L, Bisogno T, Davis JB, Pertwee RG, Di Marzo V. Overlap between the ligand recognition properties of the anandamide transporter and the VR1 vanilloid receptor: inhibitors of anandamide uptake with negligible capsaicin-like activity.FEBS Lett · 2000 · 483(1):52-6doi:10.1016/s0014-5793(00)02082-2 · PMID 11033355

    AM404, arvanil, olvanil and linvanil activate the human receptor at concentrations 400 to 10,000-fold lower than those needed to inhibit anandamide transport. A methodological caution: tools used to probe endocannabinoid transport hit TRPV1 first.

29. 29
    Gavva NR, Bannon AW, Hovland DN, Lehto SG, Klionsky L, Surapaneni S, et al. Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade.J Pharmacol Exp Ther · 2007 · 323(1):128-37doi:10.1124/jpet.107.125674 · PMID 17652633

    The preclinical characterization of AMG 517: complete antagonism of capsaicin, proton and heat activation in vitro, block of capsaicin-induced flinch in rats, and transient hyperthermia in rodents, dogs and monkeys suppressed by acetaminophen and attenuated by repeated dosing. Authored by Amgen; it correctly anticipated the problem and underestimated it.

30. 30
    Tamayo N, Liao H, Stec MM, Wang X, Chakrabarti P, Retz D, et al. Design and synthesis of peripherally restricted transient receptor potential vanilloid 1 (TRPV1) antagonists.J Med Chem · 2008 · 51(9):2744-57doi:10.1021/jm7014638 · PMID 18386885

    Analogs of AMG 517 restricted to the periphery still increased core body temperature at target-covering doses, demonstrating that the site of action for antagonist-induced hyperthermia lies outside the blood-brain barrier. Rat telemetry; an Amgen chemistry paper closing off one escape route.

31. 31
    Gavva NR. Body-temperature maintenance as the predominant function of the vanilloid receptor TRPV1.Trends Pharmacol Sci · 2008 · 29(11):550-7doi:10.1016/j.tips.2008.08.003 · PMID 18805596

    The review that reframed the channel: agonists cause hypothermia, antagonists cause hyperthermia from rodents to primates, and the thermoregulatory role is evolutionarily conserved and intimately linked to the analgesic one. Written by the Amgen scientist whose compound had just failed, and openly hopeful that other antagonists would manage the problem.

32. 32
    Huang YZ, Ma JX, Bian YJ, Bai QR, Gao YH, Di SK, et al. TRPV1 analgesics disturb core body temperature via a biased allosteric mechanism involving conformations distinct from that for nociception.Neuron · 2024 · 112(11):1815-1831.e4doi:10.1016/j.neuron.2024.02.016 · PMID 38492574

    The modern reversal of the 2008 verdict. Four antagonists compared in wild-type and mutant mice; the segment 4 to 5 linker region exposed to the vanilloid pocket is critical for hyperthermia, while proton gating, route of administration, hair coverage, temperature rhythm and inflammation were not. PSFL2874, which does not bind there, was effective against inflammatory pain without changing core temperature. Mouse data; no human trial.

33. 33
    Derry S, Sven-Rice A, Cole P, Tan T, Moore RA. Topical capsaicin (high concentration) for chronic neuropathic pain in adults.Cochrane Database Syst Rev · 2013 · (2):CD007393doi:10.1002/14651858.CD007393.pub3 · PMID 23450576

    Six studies, 2,073 participants, with 0.04 percent capsaicin as control to maintain blinding. Postherpetic neuralgia, four studies and 1,272 participants: number needed to treat 8.8, interval 5.3 to 26, at eight weeks and 7.0, interval 4.6 to 15, at twelve. HIV neuropathy, two studies and 801 participants: 5.8, interval 3.8 to 12, at twelve weeks in a single study. Serious adverse events 4.1 against 3.2 percent. Also the source for the patch being about 100 times the concentration of conventional creams.

34. 34
    Brown DC, Agnello K, Iadarola MJ. Intrathecal resiniferatoxin in a dog model: efficacy in bone cancer pain.Pain · 2015 · 156(6):1018-1024doi:10.1097/j.pain.0000000000000115 · PMID 25659068

    The strongest controlled evidence for intrathecal resiniferatoxin in any species: 72 companion dogs with bone cancer pain randomized to standard analgesia alone or plus 1.2 micrograms per kilogram intrathecally. Within six weeks 78 percent of controls against 50 percent of treated dogs required unblinding for a protocol change or euthanasia, p less than 0.03, and controls required unblinding sooner, p less than 0.02. No deafferentation pain syndrome. Single-blind, veterinary.

35. 35
    Benkhraba K, Castaño JD, Beaudry F. TRPV1, endocannabinoid, and opioid systems in analgesia: molecular mechanisms and drug development strategies.Can J Physiol Pharmacol · 2026 · 104:1-11doi:10.1139/cjpp-2025-0328 · PMID 42276013

    A current review of the signaling framework: TRPV1 activation by heat, protons or endogenous lipids drives calcium influx and engages protein kinase C, protein kinase A and mitogen-activated protein kinase pathways that modulate channel phosphorylation and excitability, with anandamide acting as a dual CB1 and TRPV1 agonist. Review, including computational and docking work, not new experiments.

36. 36
    Mobasheri A, Rannou F, Ivanavicius S, Conaghan PG. Targeting the TRPV1 pain pathway in osteoarthritis of the knee.Expert Opin Ther Targets · 2024 · 28(10):843-856doi:10.1080/14728222.2024.2416961 · PMID 39450875

    Summarizes the current therapeutic position: antagonists can induce analgesia but hyperthermia and thermal hypesthesia have limited their utility, while clinical development of agonists has progressed further, with capsaicin and resiniferatoxin providing proof of concept in knee osteoarthritis. One author is an employee of Grünenthal.

37. 37
    Derry S, Lloyd R, Moore RA, McQuay HJ. Topical capsaicin for chronic neuropathic pain in adults.Cochrane Database Syst Rev · 2009 · (4):CD007393doi:10.1002/14651858.CD007393.pub2 · PMID 19821411

    Six studies and 389 participants of 0.075 percent cream gave a number needed to treat of 6.6, interval 4.1 to 17, over six to eight weeks, with a number needed to harm of 2.5 for local skin reactions; two studies and 709 participants of the 8 percent patch gave a number needed to treat of 12, interval 6.4 to 70, for at least 30 percent relief over twelve weeks. The authors note that maintenance of blinding remains a potential problem.

38. 38
    Webster LR, Tark M, Rauck R, Tobias JK, Vanhove GF. Effect of duration of postherpetic neuralgia on efficacy analyses in a multicenter, randomized, controlled study of NGX-4010, an 8% capsaicin patch evaluated for the treatment of postherpetic neuralgia.BMC Neurol · 2010 · 10:92doi:10.1186/1471-2377-10-92 · PMID 20937130

    A negative trial, reported as such. 155 patients randomized 2:1; mean pain reduction 36.5 percent against 29.9 percent, p equals 0.296, missing the primary endpoint. A post hoc analysis limited to patients with neuralgia of at least six months gave 37.6 against 23.4 percent, p equals 0.0291. Registered as NCT00068081. The post hoc result is a hypothesis, not a finding.

39. 39
    Vinik AI, Perrot S, Vinik EJ, Pazdera L, Jacobs H, Stoker M, et al. Capsaicin 8% patch repeat treatment plus standard of care (SOC) versus SOC alone in painful diabetic peripheral neuropathy: a randomised, 52-week, open-label, safety study.BMC Neurol · 2016 · 16(1):251doi:10.1186/s12883-016-0752-7 · PMID 27919222

    Phase 3 safety study, 468 patients randomized to 30-minute or 60-minute repeat patch treatment plus standard care or standard care alone for 52 weeks. Quality-of-life scores improved more with the patch, no deterioration appeared on neurological examination or sensory testing of sharp, warm, cold and vibration stimuli, and application site pain occurred in 28.2 and 29.3 percent. Conducted and authored by Astellas; registered as NCT01478607. Open-label, so efficacy claims are weak.

40. 40
    Guo Z, Xia Y, Zhang Z. Efficacy and safety of different medications compared for the treatment of postherpetic neuralgia: a network meta-analysis.Front Pharmacol · 2025 · 16:1614587doi:10.3389/fphar.2025.1614587 · PMID 40808683

    Thirty-eight randomized trials, 8,621 participants. The NGX-4010 8 percent capsaicin patch showed a statistically significant effect on pain intensity, mean difference minus 9.20, interval minus 12.0 to minus 6.60; amitriptyline plus pregabalin had the highest probability of clinical superiority. Nine of 38 studies, 24 percent, were at high risk of bias and 15 raised some concerns, which the authors state limits the conclusions.

41. 41
    Eerdekens MH, Both C, Freitas RL. Long-Term Efficacy and Safety of Treatments for Chronic Peripheral Neuropathic Pain: A Systematic Review and Narrative Synthesis.Pain Ther · 2026 · 15(5):1313-1343doi:10.1007/s40122-026-00884-1 · PMID 42671707

    Thirty-four articles covering 31 studies of at least 13 weeks against the 2025 NeuPSIG recommendations. The high-concentration capsaicin 8 percent patch was evaluated in the greatest number of long-term studies, thirteen, across several conditions, with the lowest rates of discontinuation due to adverse events. All three authors are employees of Grünenthal, which markets the patch; the synthesis is narrative, not quantitative.

42. 42
    Niven M, Inwood M, Forget P. High-concentration (8%) capsaicin patch for chronic postoperative neuropathic pain: A systematic review of randomised controlled trials.Br J Pain · 2025 · online ahead of print, 20494637251396094doi:10.1177/20494637251396094 · PMID 41281261

    The negative boundary of the patch's evidence. Of 487 studies screened to 3 July 2025, exactly one randomized trial with 46 participants met criteria, and it showed no significant improvement over an inactive placebo patch; certainty graded very low by GRADE, the Grading of Recommendations Assessment, Development and Evaluation system. Two further trials are ongoing without published results.

43. 43
    Nash MS, McIntyre P, Groarke A, Lilley E, Culshaw A, Hallett A, et al. 7-tert-Butyl-6-(4-chloro-phenyl)-2-thioxo-2,3-dihydro-1H-pyrido[2,3-d]pyrimidin-4-one, a classic polymodal inhibitor of transient receptor potential vanilloid type 1 with a reduced liability for hyperthermia, is analgesic and ameliorates visceral hypersensitivity.J Pharmacol Exp Ther · 2012 · 342(2):389-98doi:10.1124/jpet.112.191932 · PMID 22566669

    BCTP blocked the human channel with half-maximal inhibitory concentrations of 65.4 nanomolar for capsaicin and 26.4 nanomolar for low pH, was analgesic in rats from 2 milligrams per kilogram by mouth, and raised core temperature at most 0.6 degrees Celsius at 30 and 100 milligrams per kilogram, against more than 1 degree for AMG 517 at less effective doses. Rodent data from Novartis; never taken into people.

44. 44
    Thompson V, Moshirfar M, Clinch T, Scoper S, Linn SH, McIntosh A, Li Y, Eaton M, Ferriere M, Stasi K. Topical Ocular TRPV1 Antagonist SAF312 (Libvatrep) for Postoperative Pain After Photorefractive Keratectomy.Transl Vis Sci Technol · 2023 · 12(3):7doi:10.1167/tvst.12.3.7 · PMID 36917119

    Forty participants, bilateral crossover after photorefractive keratectomy: SAF312-treated eyes had visual analog pain scores 11.13 points lower at 6 hours, p equals 0.005, and 8.56 points lower averaged over 0 to 12 hours, p equals 0.017; no serious adverse events and no delay in wound healing. Several authors are Novartis employees. A small trial in one narrow local indication.

45. 45
    Miclescu A, Karlsten R, Lönnstedt I, Halldin MM, Segerdahl M. Topically applied novel TRPV1 receptor antagonist, ACD440 Gel, reduces temperature-evoked pain in patients with peripheral neuropathic pain with sensory hypersensitivity, a randomized, double-blind, placebo-controlled, crossover study.Scand J Pain · 2025 · 25(1)doi:10.1515/sjpain-2025-0011 · PMID 40711349

    Exploratory phase 2a crossover study, 14 patients, gel applied twice daily for seven days. A significant period effect forced analysis of period 1 only, a parallel comparison: pain evoked by a 40-degree-Celsius thermoroller fell with the gel, p = 0.029; evoked mechanical hyperalgesia and brush allodynia did not change significantly, p = 0.07; no treatment-induced adverse events. Sponsored by AlzeCure Pharma, with two employee authors. Fourteen patients, so a signal and not a demonstration.

46. 46
    Iadarola MJ, Mannes AJ. The vanilloid agonist resiniferatoxin for interventional-based pain control.Curr Top Med Chem · 2011 · 11(17):2171-9doi:10.2174/156802611796904942 · PMID 21671877

    The rationale paper for local resiniferatoxin: not suitable for systemic administration, but adaptable to subcutaneous, intraganglionic or intrathecal routes. Written by the National Institutes of Health group that went on to run the human trial, and explicitly framed as the preclinical underpinning of that trial.

47. 47
    Iadarola MJ, Sapio MR, Nahama A, Mannes AJ. Intraarticular resiniferatoxin, a potent TRPV1 agonist, for treatment of osteoarthritic pain.Pain Manag · 2025 · 15(10):659-670doi:10.1080/17581869.2025.2535271 · PMID 40776673

    Review of intra-articular resiniferatoxin and lower-potency intra-articular capsaicin for knee osteoarthritis, covering human and canine studies. States explicitly that phase 1 clinical results have so far been reported only in abstract form and are preliminary. One author is affiliated with Vivasor, the company developing the compound.

48. 48
    Matta C, Takács R, Jin EJ, Helyes Z, Horváth ÁI, Juhász KZ, Tóth BI, Ellis R, Mobasheri A. The rise and fall of TRPV1-targeted analgesia in osteoarthritis: a critical appraisal.Expert Opin Pharmacother · 2026 · 27(11):1179-1196doi:10.1080/14656566.2026.2696489 · PMID 42373560

    The reversal on the agonist side: intra-articular TRPV1 agonists, despite expedited regulatory designations, failed to meet their primary endpoints in phase 3 osteoarthritis trials, and the target has completed a full translational cycle without a regulatory approval. A narrative critical appraisal, not a systematic review. Declared affiliations include PharmInVivo Ltd. and Organelle Therapeutic Inc.; the senior author also co-authored the Grünenthal-affiliated 2024 review cited here as source 37.

49. 49
    Shafer SL, Teichman SL, Gottlieb IJ, Singla N, Minkowitz HS, Leiman D, Vaughn B, Donovan JF. Safety and Efficacy of Vocacapsaicin for Management of Postsurgical Pain: A Randomized Clinical Trial.Anesthesiology · 2024 · 141(2):250-261doi:10.1097/ALN.0000000000005027 · PMID 38662910

    Triple-blinded, randomized, placebo-controlled, dose-ranging trial of a local capsaicin prodrug instilled into the surgical site in 147 bunionectomy patients. At 0.30 mg/ml: pain at rest through 96 hours 33 percent lower than placebo, interval 10 to 52 percent, p = 0.005, the prespecified primary endpoint; opioid consumption 50 percent lower, interval 26 to 67 percent, p = 0.002; 26 percent opioid-free against 5 percent, p = 0.025. Sponsored by Concentric Analgesics, two of whose staff are authors.

50. 50
    Tsai EH, Gan TJ. TRPV1 Receptor Modulators: Emerging Therapies for Neuropathic, Osteoarthritic, and Perioperative Pain.CNS Drugs · 2026 · 40(4):523-534doi:10.1007/s40263-026-01272-1 · PMID 41722000

    Narrative review of the current pipeline on both sides of the target: low- and high-dose capsaicin, resiniferatoxin, CNTX-4975 and vocacapsaicin among the agonists, NEO6860, mavatrep, ACD440 and others among the antagonists. Used here for the shape of the pipeline, not for any individual trial result.

51. 51
    Derbenev AV, Monroe MJ, Glatzer NR, Smith BN. Vanilloid-mediated heterosynaptic facilitation of inhibitory synaptic input to neurons of the rat dorsal motor nucleus of the vagus.J Neurosci · 2006 · 26(38):9666-72doi:10.1523/JNEUROSCI.1591-06.2006 · PMID 16988037

    Patch-clamp recordings in rat brainstem slices: capsaicin increased action-potential-independent inhibitory input onto dorsal motor nucleus of the vagus neurons, anandamide mimicked it, vanilloid antagonists blocked it, and the pathway was indirect, through increased glutamate release driving GABA release. Rat tissue; the human relevance is an inference.

52. 52
    Kinch DC, Peters JH, Simasko SM. Comparative pharmacology of cholecystokinin induced activation of cultured vagal afferent neurons from rats and mice.PLoS One · 2012 · 7(4):e34755doi:10.1371/journal.pone.0034755 · PMID 22514663

    Cholecystokinin activation of nodose neurons was significantly attenuated but not completely blocked by the TRP channel blocker ruthenium red in rats and two mouse strains, and capsaicin sensitivity differed about tenfold between strains, with fewer BALB/c neurons responding. A caution against generalizing any single rodent strain's TRPV1 pharmacology.

53. 53
    Ritchey CR, McCune KX, Peters JH. Comparison of receptor expression and cholecystokinin signaling between left and right nodose ganglia.Am J Physiol Cell Physiol · 2026 · 330(4):C741-C751doi:10.1152/ajpcell.00932.2025 · PMID 41661036

    NanoString messenger RNA profiling and calcium imaging: the left nodose ganglion showed greater expression of TRP channel genes including TRPV1 and more TRPV1-positive afferents, while the right showed greater cholecystokinin-1 receptor expression and stronger calcium responses to the hormone. Rodent; evidence that vagal signaling is lateralized.

54. 54
    Allen JH, de Moore GM, Heddle R, Twartz JC. Cannabinoid hyperemesis: cyclical hyperemesis in association with chronic cannabis abuse.Gut · 2004 · 53(11):1566-70doi:10.1136/gut.2003.036350 · PMID 15479672

    The founding description. Nineteen patients identified in South Australia, nine reported after exclusions and loss to follow-up, plus one previously published case. Cannabis use preceded illness in all; cessation ended it in seven; three who rechallenged became ill again. Nine of ten displayed abnormal washing behavior during attacks. An uncontrolled case series, and the origin of everything downstream.

55. 55
    Ilyas M, Fernando A, Choriyev A, Adi M, Ashraf A. Successful Management of Refractory Cannabinoid Hyperemesis Syndrome With Topical Capsaicin: A Case Report With Proposed Mechanism of Action and Literature Review.Clin Med Insights Case Rep · 2026 · 19:11795476261470568doi:10.1177/11795476261470568 · PMID 42491286

    A 28-year-old man using about 2 grams a day of high-potency cannabis for seven years; about 2 grams of 0.1 percent capsaicin cream across the abdomen stopped retching within 10 minutes, pain fell from 8 to 2 out of 10 by 30 minutes and resolved by 90. The paper describes the TRPV1 mechanism as proposed and calls for larger controlled studies. A single case, with no control for spontaneous resolution.

56. 56
    Aziz A, Waheed T, Oladunjoye O, Oladunjoye A, Hanif M, Latif F. Topical Capsaicin for Treating Cannabinoid Hyperemesis Syndrome.Case Rep Gastrointest Med · 2020 · 2020:8868385doi:10.1155/2020/8868385 · PMID 33294233

    A 41-year-old woman with recurrent vomiting unresponsive to fluids, antiemetics and morphine, in whom topical capsaicin over the epigastrium gave what the authors call dramatic relief within 24 hours, with no recurrence after discharge and counseling on abstinence. One case, unblinded, with cannabis cessation as a co-intervention.

57. 57
    Wagner S, Hoppe J, Zuckerman M, Schwarz K, McLaughlin J. Efficacy and safety of topical capsaicin for cannabinoid hyperemesis syndrome in the emergency department.Clin Toxicol (Phila) · 2019 · 58(6):471-475doi:10.1080/15563650.2019.1660783 · PMID 31482758

    Retrospective cohort of 43 patients. Emergency department length of stay was not significantly reduced, 201 against 179 minutes, p equals 0.33; patients given capsaicin received fewer additional medications, median 4 against 3 doses, p equals 0.015, with 67 percent needing nothing further before discharge, and lower opioid use, 166.5 against 69 morphine milligram equivalents. Retrospective and unblinded, with no control for severity.

58. 58
    Ruberto AJ, Sivilotti MLA, Forrester S, Hall AK, Crawford FM, Day AG. Intravenous Haloperidol Versus Ondansetron for Cannabis Hyperemesis Syndrome (HaVOC): A Randomized, Controlled Trial.Ann Emerg Med · 2020 · 77(6):613-619doi:10.1016/j.annemergmed.2020.08.021 · PMID 33160719

    Triple-blind randomized trial, 33 enrolled and 30 treated, haloperidol 13 and ondansetron 17, mean age 29, reported use 1.5 grams a day since age 19. Haloperidol was superior by 2.3 centimeters on the combined pain and nausea scales, interval 0.6 to 4.0, p equals 0.01, with less rescue medication, 31 against 59 percent, and shorter time to departure, 3.1 against 5.6 hours, p equals 0.03. Two return visits for acute dystonia, both at the higher haloperidol dose. Small, single-center.

59. 59
    Richards JR, Gordon BK, Danielson AR, Moulin AK. Pharmacologic Treatment of Cannabinoid Hyperemesis Syndrome: A Systematic Review.Pharmacotherapy · 2017 · 37(6):725-734doi:10.1002/phar.1931 · PMID 28370228

    Sixty-three articles and 205 subjects graded by Oxford levels of evidence: four level-2 studies with 64 subjects, three level-3 with 43, twelve level-4 case series with 54, and 44 level-5 case reports with 44. Benzodiazepines, then haloperidol and capsaicin, were most frequently reported effective acutely, tricyclic antidepressants for long-term treatment, and hot showers and baths were cited as universally effective in all level-4 and level-5 articles.

60. 60
    Hsu J, Kashyap S, Hurd C, McCormack L, Herrmann Z, Schwartz AC, Jackson J, DeMoss D. Treatment of cannabinoid hyperemesis syndrome: A systematic review and treatment algorithm for consultation-liaison psychiatrists.Gen Hosp Psychiatry · 2025 · 97:185-191doi:10.1016/j.genhosppsych.2025.10.012 · PMID 41129865

    Thirty-four eligible articles and 63 individual cases with patient-level data. Capsaicin cream, antipsychotics and benzodiazepines were reported to improve symptoms more effectively than promethazine, ondansetron or metoclopramide. The authors state the limitation plainly: the algorithm rests on case reports. The literature search ran only to July 2021, despite 2025 publication. PROSPERO CRD42021254888.

61. 61
    Andrews CN, Rehak R, Woo M, Walker I, Ma C, Forbes N, et al. Cannabinoid hyperemesis syndrome in North America: evaluation of health burden and treatment prevalence.Aliment Pharmacol Ther · 2022 · 56(11-12):1532-1542doi:10.1111/apt.17265 · PMID 36307209

    Administrative health data for Alberta, population about five million. Emergency department visits for vomiting with a concurrent cannabis code rose from 15 per 100,000, interval 14 to 17, before recreational legalization, to 21, interval 20 to 23, after, to 32, interval 31 to 35, in the first pandemic year, p less than 0.001, reaching 6 per 1,000 among chronic users aged 16 to 24. The accompanying survey of 157 found universal problematic use and 59 and 68 percent screening for moderate or worse anxiety and depression. Coded administrative data cannot confirm the diagnosis in any individual case.

62. 62
    Meltzer AC, Morrison C, Loganathan A, Shahamatdar S, Moon A, Heidish R, et al. Cannabinoid Hyperemesis Syndrome Is Associated With High Disease Burden: An Internet-Based Survey.Ann Emerg Med · 2025 · 85(6):521-525doi:10.1016/j.annemergmed.2025.01.008 · PMID 39985554

    An internet survey of 1,052 people with self-reported cannabinoid hyperemesis syndrome recruited through an online support group, reporting frequent use, heavy emergency department and hospital utilization, and associations with heavy daily use and with use starting in adolescence. Self-selected and self-diagnosed; it measures burden in a support-group population, not prevalence.

63. 63
    Ballaman J, Reiff C, Voirol U. [Management of cannabinoid hyperemesis syndrome].Rev Med Suisse · 2026 · 22(956):632-634doi:10.53738/REVMED.2026.22.956.47892 · PMID 41923507

    A short French-language clinical review with a case. Cited here specifically because its abstract states that the syndrome affects approximately one-third of cannabis users, a figure no population dataset supports and which conflicts with the administrative prevalence of about 6 emergency visits per 1,000 chronic users aged 16 to 24; it is included as an example of a claim that should not be repeated.

64. 64
    Sigal A, Padilla G, Carroll T, Mautone SG. Cannabinoid Hyperemesis Syndrome in Adolescents: The Role of Aprepitant as a New Treatment Option for Rapid Symptom Relief.J Adolesc Health · 2025 · 77(6):1223-1225doi:10.1016/j.jadohealth.2025.08.004 · PMID 40965394

    Two adolescents meeting Rome IV criteria, unresponsive to ondansetron, metoclopramide, haloperidol and capsaicin over 36 to 45 hours, given the neurokinin-1 antagonist aprepitant: vomiting resolved within one hour, fluids within one to two hours, solids within four to eight, both discharged the next day. Two patients, unblinded, no control group, with cannabis cessation as a co-intervention.

65. 65
    Kelly CM, Attridge M, Katsogridakis YL, Mrozek L, Jackson K, Hoffmann JA. Implementation of a Standardized Management Algorithm for Cannabis Hyperemesis Syndrome in a Pediatric Emergency Department.Pediatr Emerg Care · 2026 · 42(7):535-543doi:10.1097/PEC.0000000000003588 · PMID 42003265

    Single-center before-and-after study, July 2020 to July 2024, 533 encounters screened and 128 included from 44 adolescents. After the algorithm, capsaicin administration rose from 2.7 to 22.2 percent and metoclopramide from 6.8 to 42.6 percent, both p < 0.001, while mean haloperidol dose fell from 2.7 to 1.0 mg, p = 0.014. Adjusted odds of admission 0.57, interval 0.17 to 1.86, and length of stay unchanged. Retrospective and uncontrolled; it measures practice change, not patient benefit.

66. 66
    Merino S, Tordera L, Jun A, Yang S. Mitigating the Risk of QTc Prolongation When Using Haloperidol for Acute Treatment of Cannabinoid Hyperemesis Syndrome in Adolescents and Young Adults.J Clin Med · 2024 · 14(1):163doi:10.3390/jcm14010163 · PMID 39797246

    Two adolescent cases with QT prolongation, 528 milliseconds in a 15-year-old with hypokalemia and hypomagnesemia and 476 in a 17-year-old, plus a literature search through October 2024 identifying five severe cases of QT prolongation or torsades de pointes in adolescents and young adults. The authors recommend electrocardiographic monitoring and name topical capsaicin among the alternatives when risk is elevated.

67. 67
    Brown JM, Wilsey MJ, Dhana L, Lonsdale H. Acute Treatment of Adolescent Cannabinoid Hyperemesis Syndrome With Haloperidol, Lorazepam, and/or Capsaicin: A Single Institution Case Series.J Psychiatr Pract · 2023 · 29(5):354-358doi:10.1097/PRA.0000000000000732 · PMID 37678364

    Six adolescents at one children's hospital. Four given 5 milligrams intravenous haloperidol with 2 milligrams intravenous lorazepam, and one given haloperidol with peri-umbilical 0.025 percent topical capsaicin, had full acute relief; one treated with topical capsaicin alone improved but had persistent nausea. Six patients, no control, one institution.

68. 68
    Citak A, Kilinc E, Torun IE, Ankarali S, Dagistan Y, Yoldas H. The effects of certain TRP channels and voltage-gated KCNQ/Kv7 channel opener retigabine on calcitonin gene-related peptide release in the trigeminovascular system.Cephalalgia · 2022 · 42(13):1375-1386doi:10.1177/03331024221114773 · PMID 35861239

    Rat hemiskull with dura, trigeminal ganglion and trigeminal nucleus caudalis preparations. Agonists of TRPV1, TRPV4, TRPA1 and TRPM8 each induced calcitonin gene-related peptide release measured by enzyme-linked immunosorbent assay, and each was blocked by its specific antagonist. In vitro rat pharmacology; no clinical inference is justified from it alone.

69. 69
    Carreño O, Corominas R, Fernández-Morales J, Camiña M, Sobrido MJ, Fernández-Fernández JM, Pozo-Rosich P, Cormand B, Macaya A. SNP variants within the vanilloid TRPV1 and TRPV3 receptor genes are associated with migraine in the Spanish population.Am J Med Genet B Neuropsychiatr Genet · 2011 · 159B(1):94-103doi:10.1002/ajmg.b.32007 · PMID 22162417

    Two-stage case-control study, 1,040 cases and 1,037 controls, 149 variants across 14 TRP genes with known brain expression. Nineteen variants reached nominal significance in the 555-sample discovery set; in the 485-sample replication set only TRPV1 rs222741 and TRPV3 rs7217270 were confirmed nominally, and none of the risk haplotypes replicated. Nominal p values without genome-wide correction.

70. 70
    Wang M, Gu Y, Meng S, Kang L, Yang J, Sun D, et al. Association between TRP channels and glutamatergic synapse gene polymorphisms and migraine and the comorbidities anxiety and depression in a Chinese population.Front Genet · 2023 · 14:1158028doi:10.3389/fgene.2023.1158028 · PMID 37303955

    251 migraine patients and 600 controls, 13 variants in nine genes. In a dominant model rs8065080 gave an odds ratio of 1.75, interval 1.09 to 2.90, p equals 0.025, and rs7217270 gave 1.63, interval 1.02 to 2.58, p equals 0.039, for migraine risk. A small candidate-gene study with uncorrected nominal significance.

71. 71
    Saper JR, Klapper J, Mathew NT, Rapoport A, Phillips SB, Bernstein JE. Intranasal civamide for the treatment of episodic cluster headaches.Arch Neurol · 2002 · 59(6):990-4doi:10.1001/archneur.59.6.990 · PMID 12056936

    Twenty-eight patients randomized 2:1, 18 to civamide, the cis isomer of capsaicin, 25 micrograms per nostril once daily for 7 days, and 10 to vehicle. Percentage decrease in headaches over treatment days 1 to 7 was minus 55.5 against minus 25.9 percent, p equals 0.03, with non-significant trends afterwards. Nasal burning occurred in 14 of 18 against 1 of 10, p equals 0.001, which compromises blinding. Cluster headache, not migraine; 28 patients.

72. 72
    López-Carrillo L, Hernández Avila M, Dubrow R. Chili pepper consumption and gastric cancer in Mexico: a case-control study.Am J Epidemiol · 1994 · 139(3):263-71doi:10.1093/oxfordjournals.aje.a116993 · PMID 8116601

    Population-based case-control study in Mexico City, 1989 to 1990, 220 incident cases and 752 randomly selected controls. Age- and sex-adjusted odds ratio 5.49, interval 2.72 to 11.06, for consumers against non-consumers, and 17.11, interval 7.78 to 37.59, for self-rated high-level consumers. The authors note that no significant trend appeared when consumption was measured as frequency per day, and call for further studies. Self-rated exposure recalled after diagnosis.

73. 73
    Trujillo Rivera A, Sampieri CL, Morales Romero J, Montero H, Acosta Mesa HG, Cruz Ramírez N, Novoa Del Toro EM, León Córdoba K. Risk factors associated with gastric cancer in Mexico: education, breakfast and chili.Rev Esp Enferm Dig · 2018 · 110(6):372-379doi:10.17235/reed.2018.5042/2017 · PMID 29843516

    Forty-six gastric cancer cases against 46 blood-bank donor controls. In the final multivariable model, capsaicin consumption above 29.9 milligrams a day gave an odds ratio of 3.77, interval 1.21 to 13.11, alongside no formal education at 17.47 and an unsuitable breakfast at 8.99. Very small, with controls drawn from blood donors and heavy residual confounding by social circumstance.

74. 74
    Yang L, Sun J, Zhao M, Xi B. Chili pepper intake and all-cause and disease-specific mortality.Int J Vitam Nutr Res · 2022 · 93(4):378-384doi:10.1024/0300-9831/a000746 · PMID 35038885

    Meta-analysis of four prospective cohorts, 564,748 participants, with 31,527 all-cause, 10,184 cardiovascular and 9,868 cancer deaths. Chili consumption against none or rare: all-cause hazard ratio 0.87, interval 0.85 to 0.90; cardiovascular 0.89, interval 0.85 to 0.93; cancer-specific 0.92, interval 0.88 to 0.97, with no significant heterogeneity. Observational and about mortality, not cancer incidence; only four cohorts.

75. 75
    Bonaccio M, Di Castelnuovo A, Costanzo S, Ruggiero E, De Curtis A, Persichillo M, et al. Chili Pepper Consumption and Mortality in Italian Adults.J Am Coll Cardiol · 2019 · 74(25):3139-3149doi:10.1016/j.jacc.2019.09.068 · PMID 31856971

    The Moli-sani cohort, 22,811 Italian adults enrolled 2005 to 2010, median follow-up 8.2 years, 1,236 deaths. Chili more than four times a week against none or rare: all-cause hazard ratio 0.77, interval 0.66 to 0.90; cardiovascular 0.66, interval 0.50 to 0.86; ischemic heart disease 0.56; cerebrovascular 0.39. Observational, with intake from a food frequency questionnaire.

76. 76
    Hashemian M, Poustchi H, Murphy G, Etemadi A, Kamangar F, Pourshams A, et al. Turmeric, Pepper, Cinnamon, and Saffron Consumption and Mortality.J Am Heart Assoc · 2019 · 8(18):e012240doi:10.1161/JAHA.119.012240 · PMID 37221812

    The Golestan Cohort in Iran, 50,045 enrolled and 44,398 analyzed, 11 years of follow-up, 5,121 deaths. Black or chili pepper consumption was associated with an overall mortality hazard ratio of 0.91, interval 0.86 to 0.98. The study found no association between any of the studied spices and cancer mortality, which is the cleanest negative on the preventive side.

77. 77
    Meral O, Alpay M, Kismali G, Kosova F, Cakir DU, Pekcan M, Yigit S, Sel T. Capsaicin inhibits cell proliferation by cytochrome c release in gastric cancer cells.Tumour Biol · 2014 · 35(7):6485-92doi:10.1007/s13277-014-1864-6 · PMID 24682934

    In vitro only, in the HGC-27 gastric cancer cell line: capsaicin reduced the half-maximal inhibitory concentration of fluorouracil several-fold, with cytochrome c release as a marker of mitochondrial apoptosis. The authors themselves describe the role of capsaicin in carcinogenesis as quite controversial and call for well-controlled epidemiologic studies. No animal or human data; the PubMed record carries no retraction notice or expression of concern as of 19 October 2026.

78. 78
    Lee JS, Kim YA, Jang YJ, Oh Y, Byun S. Dihydrocapsaicin Inhibits Epithelial Cell Transformation through Targeting Amino Acid Signaling and c-Fos Expression.Nutrients · 2019 · 11(6):1269doi:10.3390/nu11061269 · PMID 31167465

    Cell transformation assays comparing chili pepper constituents: dihydrocapsaicin, not capsaicin, showed the strongest inhibition of growth-factor-induced neoplastic transformation, acting through the p70S6K1-S6 pathway and c-Fos. One author is affiliated with Nongshim, a food company. In vitro only; the opening sentence concedes the controversy over whether chili compounds promote or prevent cancer.

79. 79
    Ni L. Temperature-induced structural changes in thermosensitive transient receptor potential channels (thermoTRPs).Front Mol Neurosci · 2026 · 19:1911902doi:10.3389/fnmol.2026.1911902 · PMID 42676760

    Review of six cryo-electron microscopy studies that captured temperature-induced open states of heat-sensitive TRP channels. The author's conclusion is the one that matters here: despite these structures, the molecular mechanism by which temperature itself activates the channels remains poorly understood.