Iron

Is iron safe?

Updated 9 Nov 2026

Short answer

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.[2]

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.[3] 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]

Sources (3)

  1. Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. 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
  3. Scientific opinion on the tolerable upper intake level for iron. EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) European Food Safety Authority, 2024. efsa.onlinelibrary.wiley.com

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?Does iron help with tiredness?

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