Built for her biology,
not his marketing.
The evidence-based, non-commercial guide to supplements and claims marketed at women.
Iron
One of the clearest sex-based nutrient gaps that exists — and one of the few where the standard drugstore dose is arguably wrong for a lot of women.
The RDA for menstruating adult women — more than double the 8mg/day for adult men, entirely because of monthly menstrual blood loss.
Iron biomarkers measurably fluctuate across the menstrual cycle itself, with deficiency risk concentrated around and after menstruation — a pattern that doesn't exist for non-menstruating adults.
What it is, and why people take it
Iron is essential for hemoglobin, the protein that carries oxygen in red blood cells.
People take it to treat or prevent iron-deficiency anemia — symptoms include fatigue, dizziness, shortness of breath, and irritability.
Because iron is lost through blood, anyone who menstruates loses iron monthly in a way that people who don't menstruate simply don't, which is the basis for the sex difference in recommended intake.
Why this happens
Standard drugstore iron (ferrous sulfate) delivers a high dose of elemental iron but causes constipation and GI discomfort in a large share of users — a major reason people stop taking it before their levels actually recover. Multiple randomized trials, including one in pregnant women and one in menstruating women, found ferrous bisglycinate — an iron bound to the amino acid glycine — produces comparable or better increases in hemoglobin and ferritin at meaningfully lower doses, with significantly fewer GI side effects than ferrous sulfate.
A 2013 randomized trial found 25mg of ferrous bisglycinate performed as well as 50mg of ferrous sulfate in preventing iron deficiency in pregnancy — roughly half the dose, similar result, better tolerated. A large NHANES-based analysis further confirmed that iron status genuinely fluctuates across the menstrual cycle in reproductive-age women, reinforcing that timing and consistency matter, not just total dose.
What to look for instead
- Consider ferrous bisglycinate over ferrous sulfate if GI side effects have made you stop iron supplements before — the trial evidence shows comparable results at lower doses with less constipation.
- Don't self-diagnose deficiency — iron overload is possible and is not something to guess your way into; a ferritin blood test before supplementing is the responsible starting point.
- Pair with vitamin C, not calcium or coffee — vitamin C enhances absorption; calcium and caffeine both inhibit it if taken at the same time.
- Alternate-day dosing may improve absorption over consecutive daily dosing in some studies — worth discussing with a clinician if you're correcting a diagnosed deficiency.
A note on age
Iron needs are also elevated in menstruating teenagers, but iron supplementation should not be started in anyone under 18 without a blood test and a clinician's guidance — iron overload in someone who isn't actually deficient carries real risk, unlike some other supplements on this page.
Where to buy
What buying the best-seller implies
The default drugstore iron pill. Effective.
What the evidence actually shows
The highest-GI-side-effect form in head-to-head trials — a common reason people abandon iron supplementation before their levels recover.
Most popular pick
Evidence-based for women
Iron bisglycinate at the dose used in the trials above — meaningfully lower dose than standard sulfate, formulated specifically for absorption without the constipation.
Cost breakdown
The same per-serving prices, at a day.
Switching from the most popular pick to an evidence-based one costs about $6.60 more a month.
Sources 4 studies · show list
- Milman, Jønsson, Dyre, Pedersen, Larsen (2014). Ferrous bisglycinate 25mg iron is as effective as ferrous sulfate 50mg iron in the prophylaxis of iron deficiency and anemia during pregnancy. J Perinat Med. PubMed 24152889
- Bumrungpert, Pavadhgul, Piromsawasdi, Mozafari (2022). Efficacy and Safety of Ferrous Bisglycinate and Folinic Acid in the Control of Iron Deficiency in Pregnant Women: RCT. Nutrients. PMC8839493
- Chiang, Shih, Lu, Suzuki, Li, Chen, Fang (2026). Effects of Iron Supplementation during the Menstrual Period on Hemoglobin Levels and Fatigue in Female Athletes. J Physiol Investigation. ovid.com
- Aguree, Owora, Silveyra (2025). Cyclical fluctuations of iron biomarkers in women: Diagnostic implications for iron deficiency (NHANES analysis). Practical Laboratory Medicine. PMC12681887