Iron

Which form of iron is best?

Updated 9 Nov 2026

Short answer

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.[2]

Forms on the market

Supplements most often contain iron salts, such as ferrous sulfate, ferrous gluconate, ferric citrate and ferric sulfate.[1] Ferrous iron dissolves better and is absorbed better than ferric iron.[1] Other forms are heme iron polypeptides, carbonyl iron, iron amino-acid chelates and polysaccharide-iron complexes.[1] Ferrous bisglycinate is an amino-acid chelate.[2]

The forms hold different amounts of iron by weight:

  • ferrous fumarate: 33%[1]
  • ferrous sulfate: 20%[1]
  • ferrous gluconate: 12%[1]

The Supplement Facts label lists the elemental iron, the amount of iron itself, so there is no need to calculate it.[1] By WHO figures, 60 mg of elemental iron is in 300 mg of ferrous sulfate heptahydrate, 180 mg of ferrous fumarate or 500 mg of ferrous gluconate.[3]

What comparisons found

Ferrous sulfate is the standard iron tablet. In 20 trials against placebo, it raised the odds of stomach and gut side effects 2.32 times.[4]

A 2023 review compared ferrous bisglycinate with other iron supplements in 17 trials that lasted at least 4 weeks.[2] In 9 trials of pregnant women, 4 to 20 weeks of bisglycinate gave higher hemoglobin (standardized mean difference 0.54) and fewer stomach and gut side effects (incidence rate ratio 0.36).[2] Ferritin tended to be higher, but the difference was not significant.[2] In 4 trials of children, hemoglobin and ferritin did not differ.[2] The 4 trials in men and in women who were not pregnant were described but not pooled.[2]

The other forms that might cause fewer side effects than iron salts are carbonyl iron, amino-acid chelates, heme iron polypeptides and polysaccharide-iron complexes.[1]

Tablets or infusion

A doctor can also give iron into a vein. Ferrous sulfate tablets caused stomach and gut side effects 3.05 times more often, in odds, than iron given into a vein.[4] In anemia caused by chronic disease, iron given by injection or infusion raises hemoglobin more and causes fewer side effects than iron by mouth.[1]

In a trial of 225 people with heart failure and low iron, 150 mg of oral polysaccharide iron twice a day for 16 weeks did not improve exercise capacity.[5]

How it is taken matters too

In iron-depleted women, one morning dose was absorbed as well as the same amount split into two doses.[6] A dose of 60 mg every other day was absorbed better than the same dose every day.[6] Calcium might lower iron absorption, so experts suggest taking calcium and iron supplements at different times.[1]

What is still unknown

The authors of the bisglycinate review call for more trials that compare it with other iron supplements in groups other than pregnant women.[2]

Sources (6)

  1. Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Fischer JAJ, Cherian AM, Bone JN, Karakochuk CD Nutr Rev, 2023. PMID 36728680
  3. Daily iron and folic acid supplementation during pregnancy. World Health Organization, 2024. www.who.int
  4. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ PLoS One, 2015. PMID 25700159
  5. Effect of Oral Iron Repletion on Exercise Capacity in Patients With Heart Failure With Reduced Ejection Fraction and Iron Deficiency: The IRONOUT HF Randomized Clinical Trial. Lewis GD, Malhotra R, Hernandez AF, McNulty SE, Smith A, Felker GM, et al. JAMA, 2017. PMID 28510680
  6. 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. Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, et al. Lancet Haematol, 2017. PMID 29032957

More about iron

What is iron?Which foods contain iron?How long does it take for iron supplements to work?What is iron used for?What are the signs of iron deficiency?Is iron safe?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.