Iron Supplements: Who Needs Them, Which Form & How Much (2026)
Iron helps red blood cells carry oxygen. Most people get enough from food; women with periods are the most likely to run low. In the CDC’s latest blood tests, about 1 in 5 women aged 18 to 49 had used-up iron stores. Get a ferritin test before you take iron: it helps people who are low, and your body loses very little iron, so extra can build up.
What iron does
- Carries oxygen. Most of the iron in your body is in hemoglobin, the protein in red blood cells that carries oxygen. Much of the rest is stored as ferritin, mostly in the liver, spleen and bone marrow (NIH).
- Low iron comes in stages. First the stores run down, then, once they are used up, hemoglobin falls: that is iron-deficiency anemia. It can cause weakness, tiredness and trouble concentrating (NIH).
- How much you need. 8 mg a day for men and women over 50; 18 mg for women 19 to 50; 27 mg in pregnancy, from food and supplements together (NIH). How much iron, by age and sex.
- Food first. Meat and seafood hold heme iron, the kind the body absorbs best; beans, nuts, vegetables and fortified grains hold plant iron (NIH).
Who is likely to need iron
- Women with heavy periods. Blood loss each month uses up iron. Heavy bleeding may be behind a third or more of low iron in women of childbearing age (NIH).
- Pregnant women. Needs rise to 27 mg a day (NIH). Ask your doctor or midwife whether your prenatal vitamin covers it.
- Frequent blood donors. About 25% to 35% of regular donors develop iron deficiency (NIH).
- Vegetarians and vegans. Plant iron is absorbed less well, so needs are 1.8 times higher (NIH).
- People with gut conditions, cancer or heart failure. These can lower iron intake or absorption (NIH).
- Men, and women past menopause, found to be low. This often comes from blood loss, most often in the gut. See a doctor to find the cause, usually while you start iron (BSG 2021; AGA 2020).
In the CDC’s 2021 to 2023 blood tests, 21.7% of women aged 18 to 49 had used-up iron stores (ferritin under 15, adjusted for inflammation; our analysis). How common low iron and other deficiencies are.
Test, don’t guess
The usual tests are hemoglobin and ferritin; ferritin shows your stores. Iron is not a general energy booster, and taking it for years when you are not low can build up, because the body loses very little iron. Some people, such as those with hereditary hemochromatosis, absorb too much iron already (NIH). Signs of low iron and what the tests mean, and what the ferritin ranges mean.
Which form: standard or gentle
Doctors’ guidelines say no form of iron works better than another (AGA 2024). The standard choice is ferrous sulfate: one 325 mg tablet gives 65 mg of iron and costs a few cents. It is also the harder form on the stomach: in 20 trials against a placebo, it more than doubled the odds of side effects such as nausea and constipation (Tolkien 2015).
If iron upsets your stomach, first try taking it every other day; UK guidelines suggest this when a daily tablet is not tolerated (BSG 2021). Gentler forms, such as iron bisglycinate, caused fewer stomach complaints in trials in pregnant women (Fischer 2023), but cost more for the same iron. Ferrous sulfate vs bisglycinate.
How long iron takes to work
Hemoglobin usually starts to rise within 2 weeks of starting iron, and UK guidelines advise checking for that rise in the first 4 weeks. Refilling the stores takes longer: the same guidelines advise continuing iron for about 3 months after hemoglobin is back to normal (BSG 2021). Ask your doctor when to recheck your blood.
Iron for women
Women 19 to 50 need 18 mg a day, more than twice what men need, because of monthly blood loss (NIH). Heavy periods raise the risk most. In pregnancy, needs rise to 27 mg. After menopause, needs drop to 8 mg, the same as men, and most women get that from food. If you are low after menopause, see a doctor to find out why.
Side effects and cautions
- Stomach upset is the most common side effect. NIH says doses of 45 mg a day or more can cause nausea and constipation; in trials, ferrous sulfate more than doubled the odds of stomach side effects against a placebo (Tolkien 2015).
- The upper limit is 45 mg a day for healthy adults (NIH). Treatment doses above it are taken when a test shows low iron. More on the limit.
- Keep iron away from children. Accidental iron overdose is a leading cause of fatal poisoning in children under 6, as the warning on iron tablets and capsules says.
- Medicines. Iron can make levothyroxine and levodopa work less well, and acid-reducing PPIs can lower iron absorption (NIH). Take calcium at a different time (NIH). When to take iron.
Which iron to buy
Buy iron that is on a public testing list. 8 of the 33 iron supplements we score are on one. Our pick for low iron, Nature Made Iron 65 mg, is at the top of the page. By need:
- Buy Thorne Iron Bisglycinate 25 mg (60 capsules) on Amazon
- Buy Klean Athlete Klean Iron 27 mg (90 tablets) on Amazon
- Buy OLLY Delightful Iron 18 mg chewable (60 tablets) on Amazon
See all 33 we score, with each score, cost per 60 mg and testing list.
Iron guides
Do I need it
How much and when
Which form
Which product
How we checked this
Products. We read 38 iron products and could score 33. Scores rebuilt October 6, 2026. Every score, and why some are not scored.
Testing lists. We read NSF/ANSI 173, NSF Certified for Sport and Informed Choice in full on October 5, 2026. USP’s site and Informed Sport’s search block automated reads, so those come from our copies of September 22, 2026.
Prices. Each brand’s own list price for one pack, or a named retailer where the brand sells nothing itself. Amazon’s price can differ.
How common low iron is. Our own analysis of the CDC’s NHANES 2021 to 2023 blood tests, with the method on our deficiency page.
Sources
- NIH Office of Dietary Supplements. Iron fact sheet for health professionals (needs, food, at-risk groups, upper limit, medicines, children; updated September 4, 2025; read October 6, 2026).
- Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021. PubMed 34497146
- DeLoughery TG, et al. AGA Clinical Practice Update on Management of Iron Deficiency Anemia. Clin Gastroenterol Hepatol. 2024. PubMed 38864796. Ko CW, et al. AGA guidelines on the gastrointestinal evaluation of iron deficiency anemia. Gastroenterology. 2020. PubMed 32810434
- Tolkien Z, et al. PLoS One. 2015. PubMed 25700159. Fischer JAJ, et al. Nutr Rev. 2023. PubMed 36728680
- CDC National Center for Health Statistics, NHANES August 2021 to August 2023 (ferritin and CRP files); our analysis, method on the deficiency page.
This page is for information. It is not medical advice.