Quick answer

An iron supplement gives the body the mineral it uses to make hemoglobin, which carries oxygen in red blood cells. Most adults get enough iron from food. Women aged 19 to 50 need 18 mg a day against 8 mg for men, and total intake should stay under 45 mg a day.^1,2

Key facts

Women aged 19 to 50 need 18 mg of iron a day; men and adults over 50 need 8 mg.¹
Heme iron from meat, seafood and poultry is absorbed better than the nonheme iron in plant foods, fortified foods and most supplements.²
Calcium might interfere with iron absorption, so iron and calcium supplements are best taken at separate times.²
Iron tablets and capsules must carry a warning to keep them out of children's reach.²
Iron can lower the absorption of levothyroxine, levodopa and some antibiotics.^1,3

What iron does in the body

An adult's body holds 3 to 4 g of iron, most of it in hemoglobin. Much of the rest is stored as ferritin in the liver, spleen and bone marrow or held in the myoglobin of muscle.²

Iron is also needed for growth, for the development of the brain and nerves, for the everyday work of cells and for making some hormones. A hormone called hepcidin controls how much iron the gut absorbs and where it goes; after a large dose it rises, and the next day's dose is absorbed less well.^2,4

People lose only small amounts of iron each day in urine, stool, sweat and shed skin cells. Losses are greater in women who menstruate because of blood loss.²

Because daily losses are small, most people who eat a varied diet keep their stores topped up without a supplement. A short spell of low intake causes no obvious signs, because the body draws on its stores in the liver, spleen, muscles and bone marrow first.¹

What the evidence shows

Each claim below carries a grade and the amount the studies used. The strongest evidence comes from trials in women who menstruate, the group most likely to run short. Sports nutrition groups say iron helps performance only when iron is depleted, and more iron than the body needs causes harm rather than extra gains.^2,5

Needed to make hemoglobin, which carries oxygen in red blood cells, and myoglobin, which supplies oxygen to muscles*^1,2

Moderate evidence

A long-established role of the mineral, described by NIH. Iron is also needed for growth, development and making some hormones. This does not mean extra iron helps people who already get enough, and most people in the US do.

Amount studied: Recommended intake: 8 mg a day for men and for women over 50, 18 mg for women aged 19 to 50

Helps women who menstruate build hemoglobin and iron stores*^1,2,6

Strong evidence

A Cochrane review of 67 randomized trials in 8,506 women who menstruate found that daily iron raised hemoglobin (51 trials, 6,861 women, high-quality evidence) and iron stores, and made low iron less common. Women aged 19 to 50 need 18 mg a day, against 8 mg for men, because menstruation adds to their iron losses. Iron also roughly doubled reports of stomach and bowel side effects in these trials.

Amount studied: Daily iron tablets taken at least five days a week; the amount varied between trials

Helps blood donors replace the hemoglobin and iron lost with a donation*^2,7

Moderate evidence

In a randomized trial of 215 blood donors, 37.5 mg of iron a day as ferrous gluconate cut the time to recover 80% of the lost hemoglobin to about a month, against roughly 2.5 to 5 months without iron. Without iron, two thirds of donors had not rebuilt their iron stores after 24 weeks. It is a single trial, so the grade is moderate.

Amount studied: 37.5 mg a day as ferrous gluconate for 24 weeks after giving a unit of blood

May lessen tiredness in adults whose iron stores are low but whose hemoglobin is normal*^6,8,9

Moderate evidence

A review of randomized trials in adults with low iron stores and normal hemoglobin found that iron reduced self-reported tiredness (4 trials, 714 people) but did not change measured exercise capacity. In one trial, 198 tired women with ferritin below 50 mcg/L took 80 mg of iron a day as ferrous sulfate or a placebo for 12 weeks, and their fatigue scores fell 47.7% with iron against 28.8% with placebo. The effect is modest and rests on self-report.

Amount studied: 80 mg a day as ferrous sulfate for 12 weeks in one trial; amounts varied in the others

May improve endurance exercise capacity in women who menstruate, mainly when their iron is low*^5,6,8,10

Moderate evidence

A meta-analysis of 22 randomized trials in women of reproductive age found that daily iron raised maximal oxygen uptake by 2.35 mL per kg per minute and lowered heart rate at set workloads, and a Cochrane review reached the same conclusion. Only 3 of those trials were judged to have a low likelihood of bias, and in adults whose hemoglobin was normal, iron did not change measured exercise capacity. NIH describes the results as conflicting, and sports nutrition groups advise iron for performance only when iron is depleted.

Amount studied: Daily oral iron, in amounts and for lengths of time that varied between trials

Who is most likely to need more iron

NIH names these groups as the most likely to have trouble getting enough iron from food alone:^1,2

Teen girls and women who menstruate¹
Pregnant women and teens¹
Infants, especially those born early or small¹
People who give blood often¹
People who eat no meat, poultry or seafood¹

About 14% to 18% of Americans use a supplement that contains iron, from 6% of children aged 12 to 19 to 72% of pregnant women.²

How much iron you need

Iron for adults, per day^1,2
Who	Recommended	Upper limit
Men 19 and older	8 mg	45 mg
Women 19 to 50	18 mg	45 mg
Women 51 and older	8 mg	45 mg
Pregnancy	27 mg	45 mg
Breastfeeding	9 mg	45 mg

Both columns count iron from food and supplements together. People who eat no meat need more than the amounts shown, and the dosage guide covers children, teens, blood donors and athletes.²

Food sources

Lean meat, seafood and poultry, iron-fortified breakfast cereals and breads, white beans, lentils, spinach, kidney beans, peas, nuts and some dried fruit such as raisins all provide iron.¹

A serving of breakfast cereal fortified with 100% of the Daily Value has 18 mg of iron, 3 ounces of oysters 8 mg, a cup of canned white beans 8 mg, and half a cup of lentils, spinach or firm tofu about 3 mg.²

Eating plant sources with foods rich in vitamin C, such as citrus fruit, strawberries, sweet peppers, tomatoes and broccoli, or with some meat, poultry or seafood, helps the body take up their iron.¹

Supplement forms

Most iron supplements use ferrous or ferric salts, such as ferrous sulfate, ferrous gluconate, ferric citrate and ferric sulfate. Ferrous iron dissolves more readily and is better absorbed than ferric iron.^1,2

Labels list the iron itself, called elemental iron, rather than the weight of the whole compound, so the number on the Supplement Facts panel is the one to compare. A 65 mg tablet is 360% of the 18 mg Daily Value.²

Multivitamins made for women usually include 18 mg, while those for men and seniors frequently have less or none. Iron-only products are the ones that go well past the Daily Value.²

Side effects, warnings and medicines

High doses of iron supplements, especially on an empty stomach, can cause upset stomach, constipation, nausea, abdominal pain, vomiting and diarrhea. Doses of 45 mg a day or more are the most likely to cause them.^1,2

The upper limit for iron from food and supplements is 45 mg a day for anyone 14 or older and 40 mg for younger children. It was set to avoid stomach and bowel side effects, and doctors sometimes prescribe more for a while to rebuild low stores.^1,2

Accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Iron tablets and capsules must carry a warning to keep them out of children's reach and to call a doctor or poison control center after an accidental overdose.^1,2

People with hereditary hemochromatosis absorb too much iron, which builds up to toxic levels. NIH advises them to avoid iron and vitamin C supplements.^1,2

Ask your pharmacist how far apart to take them

Iron supplements can reduce how much levothyroxine the body absorbs and make it less effective. The levothyroxine label lists ferrous sulfate among products that may bind the medicine and says to take them apart; ask your pharmacist how to space them.^1,2,11,12

On the medicine's label

Ask your pharmacist or doctor first

Iron can bind levodopa and carbidopa in the gut and reduce how much the body absorbs, which can make the medicine less effective. The carbidopa and levodopa label says iron salts, including those in multivitamins, should be combined with it only with caution.^1,2,13

On the medicine's label

Your doctor may want to check on you

Proton pump inhibitors such as omeprazole and lansoprazole lower stomach acid, which can reduce how much nonheme iron is absorbed. Years of use did not deplete iron in people whose stores were normal, but people with low iron who take these medicines may respond less well to iron supplements.^1,2

Reported in studies

Ask your pharmacist how far apart to take them

Iron can keep tetracycline antibiotics such as doxycycline from being absorbed. The doxycycline label says iron-containing preparations impair the absorption of tetracyclines.³

On the medicine's label

Ask your pharmacist how far apart to take them

Iron can reduce the absorption of quinolone antibiotics such as ciprofloxacin. The ciprofloxacin label says products containing iron, calcium or zinc, multivitamins included, must be taken well apart from it, and sets out a schedule.¹⁴

On the medicine's label

Ask your pharmacist how far apart to take them

Oral iron supplements, including multivitamins with iron, lower blood levels of the medicine dolutegravir. Its label explains when they may be taken together with food and when they must be kept apart.¹⁵

On the medicine's label

Ask first if you are pregnant or planning a pregnancy
Ask first if you take any of the medicines above
Ask first if you have a stomach or bowel condition, or have had stomach or bowel surgery²
Ask first if you have been told you have anemia: its cause needs to be found, and extra iron does not help every type¹
Ask first if you are a man or past menopause and no one has told you that you need iron

Questions people ask

What does an iron supplement do for women?

In trials of women who menstruate, daily iron raised hemoglobin and iron stores. It also appeared to reduce tiredness and to help endurance exercise, mainly in women whose iron was low, and it roughly doubled stomach and bowel side effects.⁶

How long does an iron supplement take to work?

In blood donors taking 37.5 mg a day, hemoglobin recovered within about a month, but iron stores took a median of 76 days to rebuild. In a trial in tired women with low iron, the effect on tiredness was measured after 12 weeks.^7,9

Should men take iron supplements?

Most men do not need one. Men need 8 mg a day and average 16 to 18 mg from food, and multivitamins made for men often leave iron out. People with hereditary hemochromatosis build up too much iron, so men should take extra iron only when a clinician advises it.^1,2

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.