Iron (Bisglycinate)
Iron helps your blood carry oxygen. People who are pregnant or have heavy periods are more likely to get too little, and vegetarians need more iron than people who eat meat. Too much iron can cause problems, so check with your doctor before taking it.
Iron is part of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the rest of the body. As part of myoglobin, another protein that provides oxygen, it supports muscle metabolism. The body also needs it for growth, neurological development, cell function and making some hormones. Groups more likely to get too little iron include pregnant people and people with heavy menstrual bleeding, and endurance athletes are at higher risk of low iron status. Vegetarians need more iron than people who eat meat, because iron from plant foods is absorbed less well. In a small trial in pregnant women, total digestive complaints were less frequent with ferrous bisglycinate than with ferrous sulfate, though no single complaint differed significantly; the trial didn't measure absorption.
Research & evidence
Strong support Some support Weak or no supportColours show how well the sources back each point. The reason is next to each one.
Iron deficiency, even before anemia develops, is associated with fatigue and impaired cognitive function. Its effect on exercise capacity specifically is less settled — NIH ODS's own sports-performance fact sheet describes the evidence there as conflicting.
Moderate evidenceSome support. The source hedges (“may” or “associated with”)
Study details
The StatPearls source confirming fatigue and cognitive impairment is now flagged by NCBI itself as "provided for historical reference only... may be out of date" (archived status). NIH ODS's Exercise and Athletic Performance fact sheet states explicitly: "there is conflicting evidence on whether milder iron deficiency without anemia impairs exercise performance".
Source: NCBI Bookshelf/StatPearls, "Iron-Deficiency Anemia" (archived, historical reference status per NCBI); and NIH Office of Dietary Supplements, Dietary Supplements for Exercise and Athletic Performance — Health Professional Fact Sheet. View source
In a small trial of healthy pregnant women, black stools were reported less often with ferrous bisglycinate than with ferrous sulfate (8% vs 31% of interview responses), at doses the authors describe as equipotent. No single digestive complaint, such as constipation or nausea, differed significantly, but total digestive complaints combined were significantly more frequent with ferrous sulfate. The authors concluded that low-dose iron supplementation appears to have no clinically significant digestive side effects, since no woman needed a lower dose, a different form or to stop.
Limited evidenceSome support. Based on one or a few small trials
Study details
Milman & Bergholt 2024 report two randomized, double-blind trials in healthy pregnant Danish women. In the head-to-head trial (78 women), black stools were reported in 8% of interview responses on ferrous bisglycinate 25 mg/day vs 31% on ferrous sulfate 50 mg/day (p = 0.0002). Individual complaints did not differ significantly between the two groups, but total combined digestive complaints were significantly more frequent on ferrous sulfate (p = 0.042; black stools counted separately). A separate trial (404 women) tested four doses of ferrous fumarate; at 40 mg/day, 22% of responses reported black stools. The study calls these "equipotent" doses (equally effective clinically), never claims equivalent absorbed iron, and no pharmacokinetic/absorption data appear in the paper. A separate large meta-analysis (Tolkien et al. 2015, 43 trials, 6,831 adults) confirms ferrous sulfate significantly increases GI risk vs. placebo (OR 2.32) and IV iron (OR 3.05), but includes no bisglycinate comparison arm. The authors concluded that low-dose iron of all three kinds caused no clinically significant GI side effects (no woman needed a lower dose, a different product or to stop) — so this is a difference in degree, not severe vs. tolerable.
Source: Milman NT, Bergholt T. "Low-dose prophylactic oral iron supplementation (Ferrous Fumarate, Ferrous Bisglycinate, and Ferrous Sulphate) in pregnancy is not associated with clinically significant gastrointestinal complaints." J Pregnancy. 2024;2024:1716798; and Tolkien Z, et al. "Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis." PLoS One. 2015. View source
Women with heavy menstrual bleeding (menorrhagia) have an increased risk of iron deficiency, and endurance athletes are at increased risk of low iron status. People who follow vegetarian diets need 1.8 times as much iron as people who eat meat, because iron from plant foods (non-heme iron) is absorbed less well.
Strong evidenceStrong support. Stated plainly by NIH or an FDA label
Study details
NIH ODS (Iron fact sheet, updated September 4, 2025): "Women of reproductive age who have menorrhagia, or abnormally heavy bleeding during menstruation, have an increased risk of iron deficiency." and "The requirement for iron is 1.8 times higher for people who follow vegetarian diets than those for people who include animal products in their diet, such as meat." Hinton 2014 (a narrative review): "Endurance athletes are at increased risk for suboptimal iron status because of the combination of increased iron needs and inadequate dietary intake."
Source: NIH Office of Dietary Supplements, Iron — Health Professional Fact Sheet (updated September 4, 2025); and Hinton PS. "Iron and the endurance athlete." Appl Physiol Nutr Metab. 2014 (PubMed 25017111). View source
Dosage
| Form | Amount |
|---|---|
| Any form | Recommended daily amounts: 8 mg for men 19 and older and women 51 and older; 18 mg for women 19–50; 27 mg during pregnancy. The upper limit for adults is 45 mg a day. Higher doses to treat a diagnosed deficiency should come from your healthcare provider. |
Side effects
High doses can cause constipation, nausea and stomach pain, and taking iron with food can help. Iron can also turn stools dark. Keep iron supplements away from children: an overdose can be deadly for them.
Well documented Reported (case reports, anecdotal or one study) None reportedThe circle shows how well documented an effect is, not how serious it is. A rarely reported effect can still be serious, so always read ‘When to stop’.
| Effect | Frequency | When to stop |
|---|---|---|
| ConstipationWell documentedListed plainly by NIH, MSK or LiverTox | With high doses | Taking iron with food can help; talk to a doctor if it persists. |
| Nausea, stomach upset or pain, vomiting or diarrheaWell documentedListed plainly by NIH, MSK or LiverTox | With high doses | Taking iron with food can help; stop and talk to a doctor if it's severe or persists. |
| Dark (black) stoolsReportedReported in one study | In one pregnancy trial, reported in 8% of responses on ferrous bisglycinate and 31% on ferrous sulfate | Talk to a doctor if you're unsure or it worries you. |
Safety & interactions
Safety
High doses of iron supplements can cause stomach upset, constipation, nausea, stomach pain, vomiting and diarrhea; taking them with food can help. The upper limit for adults is 45 mg a day from food and supplements, based on the amounts linked to digestive side effects. Iron overdose is dangerous for young children: at least 43 children in the U.S. died from swallowing high-dose iron supplements between 1983 and 2000, so keep iron out of their reach. Hereditary hemochromatosis, caused by a gene mutation, is associated with too much iron building up in the body.
Contraindications
Hereditary hemochromatosis is associated with too much iron building up in the body. The American Association for the Study of Liver Diseases recommends that treatment for hemochromatosis include avoiding iron and vitamin C supplements. If you have another iron-overload condition, talk to your doctor before taking iron. If you're pregnant, take an iron supplement when your healthcare provider recommends it. If you take levothyroxine, certain antibiotics, levodopa or a proton pump inhibitor, see Drug interactions.
Drug interactions
Taking iron and levothyroxine at the same time can cause clinically significant reductions in levothyroxine's effectiveness in some patients (NIH ODS). The labels for some levothyroxine products advise against taking it within 4 hours of an iron supplement. A pharmacology review (Campbell & Hasinoff, 1991) lists thyroxine among drugs shown to form stable complexes with iron, but notes little is known about the effects of taking them together.
Iron forms chelation complexes with tetracycline antibiotics — including doxycycline, methacycline, and oxytetracycline — reducing their absorption and bioavailability if taken together (Campbell & Hasinoff, 1991).
Iron forms chelation complexes with ciprofloxacin, reducing its absorption if taken together (Campbell & Hasinoff, 1991). Ciprofloxacin's FDA label lists products containing calcium, iron, or zinc among the multivalent-cation products that can decrease ciprofloxacin absorption, and instructs that ciprofloxacin be administered at least 2 hours before or 6 hours after such products.
Proton pump inhibitors reduce stomach acid, which iron absorption depends on, so here the drug affects iron rather than the other way around. NIH ODS adds a useful nuance: long-term PPI use (up to 10 years) isn't linked to iron depletion or anemia in people with normal iron stores, but people who are already iron-deficient and taking a PPI can have a blunted response to iron supplementation.
Calcium might interfere with iron absorption, though NIH ODS is explicit that this effect “has not been definitively established.” Experts suggest taking calcium and iron supplements at different times of day as a precaution.
Iron supplements can reduce the absorption of levodopa (found in Sinemet and Stalevo, used for Parkinson's disease and restless leg syndrome), possibly through chelation. NIH ODS notes this evidence comes from healthy people, and that U.S. levodopa product labels carry this warning. Campbell & Hasinoff (1991) also list levodopa and carbidopa among drugs shown to form iron complexes.
Established: stated plainly by an authoritative source. Documented: supported by case reports or trial data. Theoretical: based on mechanism, animal data, or a cautious listing. Always check with your pharmacist or prescriber.
Combining with other herbs
Avoid combining with
Which form to buy
Ferrous salts (ferrous sulfate, gluconate or fumarate)
Good choiceAbsorbed better than other forms (per NIH)
Ferrous iron dissolves better and is more available to the body than ferric iron.
Ferric salts (ferric citrate, ferric sulfate)
Weaker choiceAbsorbed less well than other forms
Less available to the body than ferrous iron, because it dissolves less well.
Iron amino-acid chelates, heme iron polypeptides, carbonyl iron, polysaccharide-iron complexes
Works, with a catchMight cause fewer stomach side effects (limited evidence)
NIH says these forms might cause fewer stomach side effects than ferrous or ferric salts.
Ferrous bisglycinate
Works, with a catchMight cause fewer stomach side effects (limited evidence)
In one small pregnancy trial, total digestive complaints were less frequent than with ferrous sulfate, though no single complaint differed.