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Iron

Evidence Level
Very Strong
4 Clinical Trials
5 Documented Benefits
5/5 Evidence Score

Iron is an essential mineral needed to make hemoglobin, the protein in red blood cells that carries oxygen, and myoglobin in muscle. The RDA is 8 mg a day for adult men and for women over 50, 18 mg for women aged 19 to 50 and 27 mg in pregnancy, and the adult upper limit is 45 mg. Pregnant women, infants and young children, women with heavy periods, frequent blood donors and people eating plant-based diets are more likely to run low. In women with low iron, daily supplements raised hemoglobin, eased self-reported fatigue and improved exercise performance in trials, and in pregnancy they reduced anemia at term. Supplements commonly cause constipation and other digestive upset, and extra iron is harmful when stores are normal, so use them to correct low iron confirmed by a blood test. Vitamin C improves absorption, tea and coffee reduce it, and calcium may lower it.

Studied Dose RDA 8 mg/day for men and women over 50, 18 mg/day for women 19 to 50, 27 mg/day in pregnancy; adult upper limit 45 mg/day. Fatigue trials in women with low iron used 80 mg/day, and 60 mg every other day absorbed better than daily dosing.
Active Compound Iron, supplied as salts and chelates such as ferrous sulfate, ferrous fumarate and ferrous bisglycinate.
Deficiency information View details

Iron deficiency is a leading cause of anemia worldwide: the World Health Organization estimates that about half of the 1.62 billion cases of anemia are due to iron deficiency. Groups at higher risk include pregnant women, infants and young children, women with heavy menstrual bleeding and frequent blood donors. Iron deficiency anemia develops when iron stores run out and red blood cell production is impaired.

Common symptoms

  • Fatigue and weakness
  • Difficulty concentrating
  • Digestive upset
  • Lower exercise or work capacity
  • Trouble regulating body temperature
  • Weaker immune function
  • Restless legs, which are linked with low iron stores

At-risk groups

  • Menstruating women, especially those with heavy periods
  • Pregnant women (the RDA rises from 18 to 27 mg a day)
  • Infants and young children, especially babies born early or small
  • Vegetarians and vegans (plant iron is absorbed less well than heme iron)
  • Frequent blood donors
  • People with digestive conditions such as celiac disease, ulcerative colitis or Crohn's disease, or who have had stomach or bowel surgery
  • People with cancer or heart failure
When to see a doctor: Ongoing tiredness, weakness or trouble concentrating, especially in one of the groups above, is a reason to ask for a blood count and ferritin test. Do not begin iron supplements without testing first: extra iron is harmful when stores are normal, and iron interacts with several medications.
Explore related deficiencies →
Sources: NIH ODS: Iron Health Professional Fact Sheet · PMID 33285139: Iron deficiency (Pasricha 2021, Lancet) · PMID 29425576: Iron treatment of restless legs syndrome (Allen 2018, Sleep Med)

Benefits

Oxygen transport

Iron is part of hemoglobin, which carries oxygen in red blood cells, and of myoglobin, which supplies oxygen to muscle. Most of the 3 to 4 grams of iron in an adult's body is in hemoglobin.

Supported by Trial 1, Trial 2

Less fatigue when iron stores are low

In 198 menstruating women with fatigue, ferritin below 50 ug/L and no anemia, 80 mg of iron a day for 12 weeks cut fatigue scores by 47.7 percent against 28.8 percent on placebo. A meta-analysis of 4 trials in non-anemic, iron-deficient adults also found less self-reported fatigue.

Supported by Trial 1, Trial 3

Exercise performance in menstruating women

A Cochrane review of trials in menstruating women found daily iron improved maximal and submaximal exercise performance and appeared to reduce fatigue. In non-anemic, iron-deficient adults, though, a meta-analysis of 9 trials found no change in VO2max or timed exercise tests.

Supported by Trial 2, Trial 3

Attention and brain development

Iron deficiency in infancy is linked with poorer thinking, motor and social skills that persist into adolescence. In a meta-analysis of 14 trials in children over 6, teens and women, iron improved attention and concentration and raised IQ by 2.5 points only in those with anemia; the trials were small and short.

Supported by Trial 4, Trial 2

Iron needs in pregnancy

In a Cochrane review of pregnancy trials, daily iron reduced anemia at term by 70 percent and iron deficiency at term by 57 percent, based on low-quality evidence. Effects on birth weight and preterm birth were not clear.

Supported by Trial 2, Trial 3

Mechanism of action

1

Hemoglobin and myoglobin formation

Iron binds to protoporphyrin IX to form heme, which is incorporated into globin proteins to create hemoglobin and myoglobin. Each hemoglobin molecule contains four iron atoms.

2

Electron transport chain function

Iron-sulfur clusters and heme groups in Complexes I, II, III, and IV of the mitochondrial electron transport chain shuttle electrons during oxidative phosphorylation, producing the majority of cellular ATP.

3

Neurotransmitter synthesis

Iron is a cofactor for tyrosine hydroxylase (dopamine synthesis) and tryptophan hydroxylase (serotonin synthesis). In animal research, iron deficiency early in life alters brain neurotransmitter function.

Clinical trials

1
Iron for Fatigue in Non-Anemic Women with Low Ferritin
PubMed

Multicentre, closed-label, observer-blinded, placebo-controlled randomized trial of ferrous sulfate (80 mg elemental iron a day) for 12 weeks. (Vaucher et al. 2012, CMAJ)

198 menstruating women aged 18 to 53 in France with fatigue, ferritin below 50 ug/L and hemoglobin above 12.0 g/dL.

Fatigue scores fell 47.7 percent on iron against 28.8 percent on placebo (p = 0.02), with no effect on quality of life, depression or anxiety. Hemoglobin rose 0.32 g/dL and ferritin 11.4 ug/L more than on placebo.

2
Daily Iron in Menstruating Women
PubMed

Cochrane systematic review of 67 randomized and quasi-randomized trials of daily oral iron. (Low et al. 2016, Cochrane Database of Systematic Reviews)

8506 menstruating women.

Iron lowered the risk of anemia (RR 0.39) and iron deficiency (RR 0.62), raised hemoglobin, improved maximal and submaximal exercise performance and appeared to reduce fatigue. Constipation and loose stools were about twice as common. Evidence on cognition was uncertain.

3
Iron, Fatigue and Physical Capacity in Non-Anemic Adults
PubMed

Systematic review and meta-analysis of 18 randomized trials of oral, intramuscular or intravenous iron. (Houston et al. 2018, BMJ Open)

1170 non-anemic, iron-deficient adults.

Iron reduced self-reported fatigue (SMD -0.38; 4 trials, 714 people) but did not change VO2max (9 trials, 235 people) or timed exercise tests. Hemoglobin and ferritin rose.

4
Iron and Cognition in Children, Teens and Women
PubMed

Systematic review and meta-analysis of 14 randomized trials of oral iron. (Falkingham et al. 2010, Nutrition Journal)

Children aged 6 and over, adolescents and women; no trials in men or older adults.

Iron improved attention and concentration regardless of starting iron status (SMD 0.59). IQ rose 2.5 points in anemic groups only, with no effect on memory, psychomotor skills or school achievement. The trials were generally small, short and methodologically weak.

Side effects and drug interactions

Common Potential side effects

Digestive side effects are common: in menstruating women, daily iron about doubled constipation and loose stools.
High doses (45 mg a day or more) may cause nausea and constipation; taking iron with food can help. Calcium may lower iron absorption, so take calcium supplements at a different time.
Supplements with 25 mg or more of iron can reduce zinc absorption.
People with hereditary hemochromatosis absorb too much iron and are at risk of overload; do not take iron unless a blood test shows you need it.
Accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Keep iron out of reach of children, and if a child swallows iron, call a doctor or poison control center immediately.

Important Drug interactions

Calcium and dairy: calcium may reduce absorption of both heme and nonheme iron; take calcium and iron supplements at different times.
Tetracycline and quinolone antibiotics: iron binds them and lowers their absorption; follow the antibiotic label on spacing.
Levothyroxine: iron reduces its absorption; labels advise not taking levothyroxine within 4 hours of iron.
Levodopa: iron supplements may reduce how much levodopa the body absorbs.
Proton pump inhibitors: by lowering stomach acid they can reduce iron absorption.
Vitamin C: enhances absorption of nonheme iron.

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Frequently asked questions about Iron

How much iron should I take?

It depends on need. The RDA is 8 mg for men and 18 mg for women aged 19 to 50 (27 mg in pregnancy). Multivitamins for women typically provide 18 mg, while many iron-only supplements provide 65 mg. Do not take high-dose iron without a blood test confirming you need it, since excess iron is harmful.

What is the best way to absorb iron?

Take iron on an emptier stomach, ideally in the morning, and keep coffee and tea about an hour away: in one study, coffee taken with iron cut absorption by about half. Calcium may lower absorption, so take calcium supplements at a different time. Vitamin C modestly raised absorption in a small study, but in a randomized trial of 440 adults with low iron, adding it did not meaningfully speed up the rise in hemoglobin, so it is optional. If iron upsets your stomach, taking it with a little food can help.

Why does iron upset my stomach?

Iron commonly causes constipation, nausea and stomach upset, more so at doses of 45 mg a day or more, and taking it with food can help. Dark or black stools are also common: in a study of pregnant women on low-dose iron, 8 to 31 percent reported them, depending on the iron form and dose.

Should I take iron every day or every other day?

Each dose of 60 mg or more raises a hormone (hepcidin) that blocks absorption of the next dose for about a day, so a dose taken every other day is absorbed better. In trials, though, alternate-day and daily dosing raised iron stores and hemoglobin about equally: in a double-blind trial, the same total iron taken on alternate days gave the same ferritin with fewer stomach side effects, over twice the time. Twice-daily dosing was faster in some studies, and daily dosing is favored in pregnancy. Follow your doctor's guidance when correcting a diagnosed deficiency.

What is Iron?

Iron is an essential mineral needed to make hemoglobin, the protein in red blood cells that carries oxygen, and myoglobin in muscle. The RDA is 8 mg a day for adult men and for women over 50, 18 mg for women aged 19 to 50 and 27 mg in pregnancy, and the adult upper limit is 45 mg.

What is Iron used for?

Iron is researched primarily for Energy and Athletic Performance. Iron is part of hemoglobin, which carries oxygen in red blood cells, and of myoglobin, which supplies oxygen to muscle. Most of the 3 to 4 grams of iron in an adult's body is in hemoglobin.

What are the signs of Iron deficiency?

Iron deficiency is a leading cause of anemia worldwide: the World Health Organization estimates that about half of the 1.62 billion cases of anemia are due to iron deficiency. Groups at higher risk include pregnant women, infants and young children, women with heavy menstrual bleeding and frequent blood donors.

Is Iron safe, and does it have side effects?

Side effects and cautions reported for Iron include: Digestive side effects are common: in menstruating women, daily iron about doubled constipation and loose stools. High doses (45 mg a day or more) may cause nausea and constipation; taking iron with food can help. It may also interact with some medications. Iron is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Iron interact with any medications?

Possible interactions include: Calcium and dairy: calcium may reduce absorption of both heme and nonheme iron; take calcium and iron supplements at different times. Tetracycline and quinolone antibiotics: iron binds them and lowers their absorption; follow the antibiotic label on spacing. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Iron?

NutraSmarts rates the evidence for Iron as Very Strong (5 out of 5). It is backed by 4 study summaries and 13 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(13 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247-54. doi: 10.1503/cmaj.110950.PubMedUsed to support: Multicentre, closed-label, observer-blinded randomized trial in 198 French women aged 18 to 53 with fatigue, ferritin below 50 ug/L and haemoglobin above 12.0 g/dL. Oral ferrous sulfate (80 mg elemental iron a day) for 12 weeks lowered fatigue scores by 47.7 percent against 28.8 percent on placebo (p = 0.02), with no effect on quality of life, depression or anxiety. Haemoglobin rose 0.32 g/dL and ferritin 11.4 ug/L more than on placebo.
  2. Allen RP, Picchietti DL, Auerbach M, Cho YW, Connor JR, Earley CJ, Garcia-Borreguero D, Kotagal S, Manconi M, Ondo W, Ulfberg J, Winkelman JW, International Restless Legs Syndrome Study Group (IRLSSG). Evidence-based and consensus clinical practice guidelines for the iron treatment of restless legs syndrome/Willis-Ekbom disease in adults and children: an IRLSSG task force report. Sleep Med. 2018;41:27-44. doi: 10.1016/j.sleep.2017.11.1126.PubMedUsed to support: IRLSSG task force review of 31 studies of iron for restless legs syndrome. Intravenous ferric carboxymaltose (1000 mg) was judged effective for moderate to severe RLS with ferritin below 300 ug/L; oral iron (65 mg elemental iron) was judged possibly effective when ferritin is 75 ug/L or less. Evidence in children was insufficient.
  3. Peña-Rosas JP, De-Regil LM, Garcia-Casal MN, Dowswell T. Daily oral iron supplementation during pregnancy. Cochrane Database Syst Rev. 2015;2015(7):CD004736. doi: 10.1002/14651858.CD004736.pub5.PubMedUsed to support: Cochrane review of 61 trials (44 contributing data, 43,274 women). Daily oral iron in pregnancy reduced maternal anaemia at term by 70 percent (RR 0.30) and iron deficiency at term by 57 percent (RR 0.43), both low-quality evidence. Effects on low birthweight and preterm birth were not clear.
  4. Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, Moretti D, Zimmermann MB. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533. doi: 10.1016/S2352-3026(17)30182-5.PubMedUsed to support: Two open-label randomized trials with isotope-labelled ferrous sulfate in iron-depleted Swiss women. In 40 women, 60 mg given on alternate days for 28 days gave higher cumulative fractional absorption (21.8 vs 16.3 percent) and more total iron absorbed (175.3 vs 131.0 mg) than the same 14 doses on consecutive days, with lower hepcidin. In 20 women, splitting 120 mg into two daily doses raised hepcidin without increasing absorption.
  5. Lozoff B, Beard J, Connor J, Barbara F, Georgieff M, Schallert T. Long-lasting neural and behavioral effects of iron deficiency in infancy. Nutr Rev. 2006;64(5 Pt 2):S34-43; discussion S72-91. doi: 10.1301/nr.2006.may.s34-s43.PubMedUsed to support: Review of follow-up studies: iron deficiency in infancy is linked with poorer cognitive, motor and social-emotional function from preschool age to adolescence, with persisting neurophysiologic differences. Animal research points to effects on brain metabolism, myelination and neurotransmitter function during development.
  6. Falkingham M, Abdelhamid A, Curtis P, Fairweather-Tait S, Dye L, Hooper L. The effects of oral iron supplementation on cognition in older children and adults: a systematic review and meta-analysis. Nutr J. 2010;9:4. doi: 10.1186/1475-2891-9-4.PubMedUsed to support: Meta-analysis of 14 randomized trials of oral iron in children aged 6 and over, adolescents and women (no trials in men or older people). Iron improved attention and concentration regardless of baseline iron status (SMD 0.59) and raised IQ by 2.5 points in anaemic groups only, with no effect on memory, psychomotor skills or school achievement. The trials were generally small, short and methodologically weak, with signs of publication bias.
  7. Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832-43. doi: 10.1056/NEJMra1401038.PubMedUsed to support: Review article on iron-deficiency anemia in the New England Journal of Medicine (no abstract on PubMed).
  8. Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. Iron deficiency. Lancet. 2021;397(10270):233-248. doi: 10.1016/S0140-6736(20)32594-0.PubMedUsed to support: Lancet seminar: iron deficiency is a leading contributor to the global burden of disease, particularly in children, premenopausal women and low- and middle-income countries; it can impair function even without anaemia. Hepcidin rises after oral iron and limits absorption of high doses. Oral iron is first-line treatment in most cases, and an underlying cause should be sought.
  9. Low MS, Speedy J, Styles CE, De-Regil LM, Pasricha SR. Daily iron supplementation for improving anaemia, iron status and health in menstruating women. Cochrane Database Syst Rev. 2016;4(4):CD009747. doi: 10.1002/14651858.CD009747.pub2.PubMedUsed to support: Cochrane review of 67 trials in 8506 menstruating women. Daily oral iron lowered the risk of anaemia (RR 0.39, moderate-quality evidence) and iron deficiency (RR 0.62), raised haemoglobin, improved maximal and submaximal exercise performance and appeared to reduce symptomatic fatigue. Gastrointestinal side effects were more common, including hard stools or constipation (RR 2.07, 8 trials, 1036 women) and loose stools (RR 2.13). Evidence on cognitive performance was uncertain. The review pooled all forms of oral iron.
  10. Houston BL, Hurrie D, Graham J, Perija B, Rimmer E, Rabbani R, Bernstein CN, Turgeon AF, Fergusson DA, Houston DS, Abou-Setta AM, Zarychanski R. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. BMJ Open. 2018;8(4):e019240. doi: 10.1136/bmjopen-2017-019240.PubMedUsed to support: Systematic review and meta-analysis of 18 randomized trials (1170 people) of oral, intramuscular or intravenous iron in non-anaemic, iron-deficient adults. Iron reduced self-reported fatigue (SMD -0.38; 4 trials, 714 participants) but did not change objective physical capacity such as maximal oxygen consumption (9 trials, 235 participants) or timed exercise tests. Haemoglobin and ferritin rose.
  11. Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999;81(4):289-95..PubMedUsed to support: Radio-iron absorption study in adults eating a bread meal. Coffee, black tea, cocoa and several herb teas all reduced non-haem iron absorption in proportion to their polyphenol content; black tea cut absorption by 79 to 94 percent, and adding milk made little difference.
  12. NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. National Institutes of Health. 2025;Online fact sheet, updated September 4, 2025..SourceUsed to support: Not PubMed-indexed (official NIH fact sheet). Gives the iron RDAs (8 mg a day for men and for women over 50, 18 mg for women 19 to 50, 27 mg in pregnancy) and the adult Tolerable Upper Intake Level of 45 mg a day; notes that supplements with 25 mg or more can reduce zinc absorption, that ferrous fumarate is 33 percent, ferrous sulfate 20 percent and ferrous gluconate 12 percent elemental iron, and that the WHO estimates about half of the 1.62 billion cases of anemia worldwide are due to iron deficiency. Lists interactions with levodopa, levothyroxine (labels advise a 4-hour gap) and proton pump inhibitors, advises taking calcium and iron supplements at different times, and quotes the required US warning that accidental iron overdose is a leading cause of fatal poisoning in children under 6.
  13. 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: Results From Two Randomized Studies. J Pregnancy. 2024;2024:1716798. doi: 10.1155/2024/1716798.PubMedUsed to support: Two randomized double-blind studies in 482 healthy pregnant women taking low-dose iron as ferrous fumarate (20 to 80 mg a day), ferrous bisglycinate (25 mg) or ferrous sulphate (50 mg). Digestive complaints did not rise with dose between 20 and 60 mg of fumarate, but 80 mg brought more constipation and laxative use. Black stools became more frequent with dose and were reported by 8 percent on bisglycinate 25 mg, 22 percent on fumarate 40 mg and 31 percent on sulphate 50 mg. No woman needed to change or stop her iron.