Not yet investigated
What we have so far
The master switch on iron absorption. One oral dose raises it for a day and shuts the door — which is why taking iron every day absorbs less than taking it every other day.
This star is on the map because the show intends to take the question apart properly — the trials, how it is actually tested, whether the thing sold is the thing studied. That work is not done yet, and nothing here should be read as a conclusion. When the investigation is published it will appear on this page.