Supplements › Vitamin

Iron

A mineral the blood needs to carry oxygen. Most studied for preventing anemia, including in pregnancy, and for tiredness and fitness in people low in iron.

Strongest evidenceEnergy & FatigueStrong, 719 studies
Median dose in studies40 mg a dayFrom 1,269 study groups
Daily need for adults8 to 18 mg a dayUS: 8 mg for men, 18 mg for women aged 19 to 50, 27 mg in pregnancy.
Upper limit45 mg a dayUS, from age 14. In the EU, a safe level of 40 mg a day.
Ask a doctor or pharmacist first

Ask a doctor or pharmacist first if you:

  • have hemochromatosis, a genetic disease that makes the body store too much iron: its treatment includes avoiding iron and vitamin C supplements[1]
  • take levothyroxine: iron lowers its absorption, and some labels advise a gap of 4 hours[1]
  • take levodopa for Parkinson's disease or restless legs: iron can lower how much of it the body absorbs[1]
  • take a proton pump inhibitor such as omeprazole: people with iron deficiency who take one can respond less well to iron[1]
  • are a man or a woman after menopause and have low iron: the usual causes are blood loss in the gut and poor absorption[2]
  • are pregnant: US expert groups disagree on routine iron for pregnant women without symptoms[1]

Keep iron supplements away from children. US labels warn that accidental overdose of iron products is a leading cause of fatal poisoning in children under 6.[1]

Overview

What is iron?

Iron is a mineral that the body needs to make hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the tissues.[1] An adult body holds 3 to 4 g of iron, and most of it is in hemoglobin.[1]

Food has two forms of iron. Heme iron comes from meat, seafood and poultry. Nonheme iron comes from plants and fortified foods.[1] Supplements most often contain iron salts such as ferrous sulfate, ferrous gluconate, ferric citrate and ferric sulfate. Other forms include carbonyl iron, iron amino-acid chelates, heme iron polypeptides and polysaccharide-iron complexes.[1]

What is iron used for?

Iron supplements are used to prevent and treat iron deficiency, the most common cause of anemia.[1] In a Cochrane review of trials in menstruating women, daily iron cut the risk of anemia by 61% (10 trials, 3,273 women) and raised hemoglobin.[3]

Other effects show up mainly in people who lack iron:

  • Tiredness: in 4 trials with 714 adults who had low iron but no anemia, iron reduced self-reported fatigue.[4] More on energy.
  • Pregnancy: a 2024 Cochrane review of 57 trials found that daily iron may reduce anemia at term (4.0% against 7.4%) and probably reduces low birthweight slightly (5.2% against 6.1%).[5]
  • Exercise: in 18 trials in women of childbearing age, iron raised VO2max, a measure of aerobic fitness, by 2.35 mL/kg/min.[6] In adults with low iron but no anemia, it did not improve measured physical capacity.[4] More on fitness.
  • Thinking in school children: in 13 studies in children aged 6 to 12, iron improved scores of intelligence, attention and memory, but not school results.[7] More on cognition.

In people with heart failure and low iron, a trial of iron by mouth found no gain in exercise capacity over 16 weeks.[8]

Is iron safe?

Adults with normal gut function have very little risk of iron overload from food.[1] High doses from supplements can cause stomach upset, constipation, nausea, abdominal pain, vomiting and diarrhea, and taking iron with food can reduce these effects.[1] In 20 trials, ferrous sulfate raised the odds of stomach and gut side effects 2.32 times compared with placebo.[9]

The US upper limit is 45 mg a day from age 14 and 40 mg for younger children.[1] EFSA could not set an upper limit and named 40 mg a day a safe level for adults, including in pregnancy.[10] Doctors sometimes prescribe more to treat iron deficiency anemia.[1]

Accidental overdose of iron products is a leading cause of fatal poisoning in children under 6.[1] People with hemochromatosis, a genetic disease that makes the body store too much iron, are advised to avoid iron supplements.[1] Iron lowers the absorption of levothyroxine and levodopa.[1]

How it works

Iron is part of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the tissues.[1] An adult body holds 3 to 4 g of iron, and most of it is in hemoglobin.[1]

  1. Gut absorption

    The gut absorbs heme iron from meat, seafood and poultry better than nonheme iron from plants and fortified foods.[1] Vitamin C and meat raise the absorption of nonheme iron. Phytate in grains and beans and polyphenols in some plant foods lower it.[1] Stomach acid also helps the body absorb nonheme iron.[1]

  2. Hepcidin control

    Hepcidin, a hormone in the blood, controls how much iron the gut absorbs and where iron goes in the body.[1] In iron-depleted women, a dose of iron every day kept hepcidin higher than a dose every other day, and a smaller share of each dose was absorbed.[11]

  3. Transport

    In the blood, a protein called transferrin binds iron and carries it through the body.[1]

  4. Oxygen supply

    Most iron goes into hemoglobin in red blood cells. Muscles hold iron in myoglobin, another protein that supplies oxygen.[1] Iron is also needed for growth, brain development and making some hormones.[1]

  5. Storage and loss

    Much of the rest is stored as ferritin and hemosiderin in the liver, spleen and bone marrow.[1] The body loses only small amounts in urine, stool, sweat and skin cells, and more with menstrual bleeding.[1] In men, the median loss is 0.95 mg a day.[12]

Explainers

Benefits and studies

Each row is one benefit. The square shows the grade and the number of studies. Open a row to see the most trusted studies.

StrongModerateWeakNo trials
BenefitMain effects in studiesStudiesPeople

Dose

In athletes, 16 to 100 mg of elemental iron a day for 6 to 8 weeks raised blood iron markers.[13] A trial in tired women with low ferritin gave 80 mg a day for 12 weeks.[14] The US recommended intake for adults is 8 mg a day for men and 18 mg for women aged 19 to 50. EFSA, the EU food safety authority, sets 11 mg for men and 16 mg for women before menopause.[1][12]

Usual dose

For tiredness in women with low ferritin, a trial in 198 women gave 80 mg a day.[14] For pregnancy, the World Health Organization (WHO) recommends 30 to 60 mg a day with 400 µg of folic acid.[15] In athletes, 16 to 100 mg a day raised blood iron markers.[13] Iron-only supplements often contain 65 mg, and multivitamins for women usually contain 18 mg.[1]

Daily need

The US recommended intake is 11 mg a day at 7 to 12 months, 7 mg at 1 to 3 years, 10 mg at 4 to 8 years and 8 mg at 9 to 13 years. At 14 to 18 years it is 11 mg for boys and 15 mg for girls. Men from age 19 and women from age 51 need 8 mg, and women aged 19 to 50 need 18 mg. In pregnancy it is 27 mg, and when breastfeeding 9 to 10 mg.[1] People who eat no meat need 1.8 times more, because the body absorbs less iron from plants.[1] EFSA sets 11 mg for men and for women after menopause, and 16 mg for women before menopause, including in pregnancy and breastfeeding.[12]

Form

Labels list elemental iron, the amount of iron itself.[1] Ferrous fumarate is 33% iron by weight, ferrous sulfate 20% and ferrous gluconate 12%.[1] Ferrous iron dissolves better and is absorbed better than ferric iron.[1]

When to take

Taking iron with food can reduce stomach side effects.[1] In iron-depleted women, one morning dose every other day was absorbed better than the same dose every day, and splitting a dose in two did not help.[11] Experts suggest taking calcium and iron supplements at different times. Some levothyroxine labels advise a gap of 4 hours from iron.[1]

How long

In adults with iron deficiency anemia who took 100 mg of iron three times a day, hemoglobin rose by 1.84 g/dL after 2 weeks.[16] In athletes, courses of 6 to 8 weeks raised blood iron markers, and shorter courses did not.[13] A fatigue trial in 198 women lasted 12 weeks.[14]

Upper limit

The US upper limit is 45 mg a day from age 14, including in pregnancy, and 40 mg for younger children.[1] EFSA could not set an upper limit. It set a safe level of 40 mg a day for adults, including in pregnancy and breastfeeding, and 10 to 35 mg for children aged 1 to 17.[10] Doctors sometimes prescribe more to treat iron deficiency anemia. People who take a medicine every day should ask a doctor or pharmacist first.[1]

Doses in studies

Most studies gave 50 to 100 mg a day, for 1 to 3 months.

The counts are study groups. A study group is one set of people in a study who took the same dose. One study can have several groups. These are amounts from studies, not advice for you.

How much a day1,269 groups in 712 studies
  1. Up to 2.5 mg48 groups
  2. 2.5 to 5 mg59 groups
  3. 5 to 10 mg164 groups
  4. 10 to 25 mg221 groups
  5. 25 to 50 mg202 groups
  6. 50 to 100 mg310 groups
  7. 100 to 250 mg126 groups
  8. 250 to 500 mg48 groups
  9. 500 to 1,000 mg64 groups
  10. 1,000 mg or more27 groups
For how long1,064 groups in 612 studies
  1. 1 week or less114 groups
  2. 1 to 4 weeks123 groups
  3. 1 to 3 months426 groups
  4. 3 to 6 months251 groups
  5. 6 to 12 months127 groups
  6. More than a year23 groups
Half of the groups took less than 40 mg a day. Half took more.Half of the groups took 12.5 to 100 mg a day. A quarter took less, a quarter took more.91 of 1269 groups took 500 mg a day or more. Do not copy a study dose without a doctor.

Taken together

567 of 1379 study groups took iron with something else. This shows what studies gave together, not what works together.

  1. Folic Acid174 studies323
  2. Zinc103 studies125
  3. Vitamin A67 studies92
  4. Vitamin C64 studies89
  5. Vitamin B1240 studies50
  6. Vitamin E30 studies42
  7. Copper30 studies39
  8. Iodine29 studies35
  9. Vitamin D29 studies35
  10. Selenium25 studies29
Bars and numbers count study groups. A companion is shown only when at least 3 different studies gave it.

Protocols

What studies used when they gave iron for one goal, or to a group of people.

Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.

Questions

12 questions in 4 groups. Open one for the short answer; each has its own page with the full answer and sources.

Basics
Which foods contain iron?

Lean meat and seafood are the richest sources of heme iron, the form the body absorbs best.[1] Beans, nuts, vegetables and fortified bread and cereals give nonheme iron.[1] A serving of fortified breakfast cereal can give 18 mg. Oysters (3 ounces) and canned white beans (1 cup) give 8 mg, beef liver (3 ounces) 5 mg, and lentils or spinach (half a cup) 3 mg.[1]

The body absorbs 14% to 18% of the iron from mixed diets with meat, seafood and vitamin C, and 5% to 12% from vegetarian diets.[1] For this reason, the US recommended intake for vegetarians is 1.8 times higher.[1] Spinach has iron, but its polyphenols lower how much of it the body takes up.[1]

Open the answer with sources ›
What are the signs of iron deficiency?

Iron deficiency develops in stages. First the iron stores fall. Then less iron reaches the cells that make red blood cells, while hemoglobin is usually still normal. Last, hemoglobin drops and iron deficiency anemia appears.[1] Anemia can cause weakness, tiredness, trouble concentrating and stomach or gut problems. It can also weaken immune function and body temperature control.[1]

A blood ferritin level below 30 µg/L suggests iron deficiency, and below 10 µg/L suggests iron deficiency anemia. Inflammation, for example from an infection, can raise ferritin.[1] Groups at higher risk include pregnant women, infants and young children, women with heavy periods, frequent blood donors, and people with cancer, gut disorders or heart failure.[1] In men and in women after menopause, the usual causes are blood loss in the gut and poor absorption, so a doctor looks for the source.[2]

Open the answer with sources ›
Benefits
Does iron help with tiredness?

Probably, in people with low iron. In a 2018 review, iron reduced self-reported fatigue in 4 trials with 714 adults who had low iron but no anemia, and the authors rated the evidence as moderate.[4] It did not improve measured physical capacity, such as VO2max.[4] In a 2025 review, the effects of iron disappeared when people with iron deficiency were left out.[17]

Read the full answer ›
Do iron supplements help in pregnancy?

Daily iron in pregnancy may reduce anemia and iron deficiency at term, and it probably lowers the share of low-birthweight babies slightly (5.2% against 6.1%).[5] It probably makes little or no difference to preterm birth and may make little or no difference to deaths of newborns.[5] WHO recommends 30 to 60 mg of iron a day with 400 µg of folic acid.[15]

Read the full answer ›
Does iron improve exercise performance?

In women of childbearing age, it can. In a 2014 review, daily iron raised VO2max by 2.35 mL/kg/min in 18 trials in these women. It also lowered heart rate at a set workload by 4 beats per minute.[6] In a 2024 review of 13 studies in athletes, iron raised ferritin, mainly in those who started at 12 µg/L or lower. The effect on VO2max was small and not significant.[13]

A review of trials in adults with low iron but no anemia found no gain in measured physical capacity.[4] In female athletes with low ferritin, endurance improved by 2% to 20% with about 100 mg of iron a day, but 18 of the 23 studies had groups of 20 athletes or fewer.[18] More on fitness.

Open the answer with sources ›
Does iron improve memory and concentration?

In school children, reviews found gains in some scores, larger in children with anemia. A 2023 review of 13 studies in children aged 6 to 12 found better scores of intelligence, attention and memory, but no change in school results.[7] The gain in intelligence was larger in children with anemia.[7] An earlier review of 32 studies in 7,089 children aged 5 to 12 also found better cognitive scores, and IQ rose by 4.55 points in children with anemia.[19] A 2022 review of 9 studies in children and teenagers in low- and middle-income countries found better intelligence scores, but no effect on attention or memory.[20]

In children aged 4 to 23 months, 6 trials found no difference in mental or motor development.[21] In menstruating women, trials on thinking gave conflicting results.[3] More on cognition.

Open the answer with sources ›
Does iron help with hair loss?

The evidence is weak. A 2006 review found that studies on iron deficiency and hair loss disagree.[2] It found not enough evidence to give iron to people with hair loss who have low iron but no anemia.[2] A 2024 review of vitamins and minerals in pattern hair loss called the evidence not entirely consistent.[22]

Open the answer with sources ›
Safety
What are the side effects of iron supplements?

High doses of iron can cause side effects in the stomach and gut: constipation, nausea, stomach pain, diarrhea and vomiting.[1] In 20 trials, ferrous sulfate raised the odds of these side effects 2.32 times compared with placebo.[9] Taking iron with food can reduce them.[1]

Read the full answer ›
Which medicines interact with iron?

Iron lowers the absorption of levothyroxine, a thyroid hormone medicine, and some of its labels advise a gap of 4 hours.[1] Iron also lowers the absorption of levodopa, a medicine for Parkinson's disease and restless legs.[1] Proton pump inhibitors such as omeprazole lower stomach acid, and people with iron deficiency who take them can respond less well to iron.[1]

Calcium might lower iron absorption, so experts suggest taking calcium and iron supplements at different times.[1] People who take a medicine every day should talk about their iron with a doctor or pharmacist.[1]

Open the answer with sources ›
Forms and timing
Which form of iron is best?

Ferrous forms, such as ferrous sulfate, are absorbed better than ferric forms.[1] In pregnant women, ferrous bisglycinate raised hemoglobin more and caused fewer stomach and gut side effects than other iron supplements. In children it made no difference, and trials in other groups were not pooled.[23]

Read the full answer ›
Is taking iron every other day better than every day?

It may be absorbed better. In iron-depleted young women, 14 doses of 60 mg taken every other day gave a higher share absorbed than 14 doses taken on consecutive days (21.8% against 16.3%).[11] A daily dose kept hepcidin, the hormone that limits absorption, higher.[11]

In a 2025 review of 11 trials with 1,014 people with iron deficiency anemia, daily and every-other-day doses raised hemoglobin about the same. The certainty of this evidence was very low.[24]

Open the answer with sources ›
Should iron be taken with vitamin C?

Vitamin C helps the body absorb nonheme iron from food.[1] With iron tablets, a 2020 trial in 440 adults with iron deficiency anemia found no extra benefit.[16] The people took 100 mg of iron three times a day, with or without 200 mg of vitamin C. After 2 weeks, hemoglobin rose by 2.00 g/dL with vitamin C and by 1.84 g/dL without, which counted as equivalent.[16] Ferritin and side effects did not differ.[16]

Open the answer with sources ›

References

  1. Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. Trost LB, Bergfeld WF, Calogeras E J Am Acad Dermatol, 2006. PMID 16635664
  3. Daily iron supplementation for improving anaemia, iron status and health in menstruating women. Low MS, Speedy J, Styles CE, De-Regil LM, Pasricha SR Cochrane Database Syst Rev, 2016. PMID 27087396
  4. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. Houston BL, Hurrie D, Graham J, Perija B, Rimmer E, Rabbani R, et al. BMJ Open, 2018. PMID 29626044
  5. Daily oral iron supplementation during pregnancy. Finkelstein JL, Cuthbert A, Weeks J, Venkatramanan S, Larvie DY, De-Regil LM, et al. Cochrane Database Syst Rev, 2024. PMID 39145520
  6. Iron supplementation benefits physical performance in women of reproductive age: a systematic review and meta-analysis. Pasricha SR, Low M, Thompson J, Farrell A, De-Regil LM J Nutr, 2014. PMID 24717371

Review history

  1. 9 Nov 2026CheckedReviewerChecked every claim against its source.
  2. 9 Nov 2026Page createdResearcherFirst edition, written from PubMed and public health sources.