What are the side effects of iron supplements?

Stomach and gut effects

High doses of iron can cause stomach upset, constipation, nausea, abdominal pain, vomiting and diarrhea.^[1] These effects can appear at 45 mg a day or more, and the US upper limit is based on them.^[1]

A 2015 review of 43 trials with 6,831 adults measured the side effects of ferrous sulfate, the standard iron tablet.^[2] Compared with placebo (20 trials), it raised the odds of stomach and gut side effects 2.32 times. Compared with iron given into a vein (23 trials), it raised them 3.05 times.^[2] The review found no link between the dose and these odds.^[2]

In a Cochrane review of trials in menstruating women, iron raised the risk of:

• constipation or hard stools: risk ratio 2.07 (8 trials)^[3]
• loose stools or diarrhea: risk ratio 2.13 (6 trials)^[3]
• any stomach or gut side effect: risk ratio 1.99 (5 trials)^[3]

It did not raise nausea, and women on iron kept taking it about as often as women in the control groups.^[3] In children aged 4 to 23 months, vomiting and fever were more common with daily iron (risk ratios 1.38 and 1.16).^[4]

Dark stools

Black stools show that large amounts of iron stayed unabsorbed in the gut. EFSA does not count them as harmful in themselves.^[5] In the trials EFSA used, black stools did not occur with 20 to 25 mg of supplemental iron a day.^[5]

What may lower side effects

• Taking iron with food.^[1]
• Another form: carbonyl iron, iron amino-acid chelates, heme iron polypeptides and polysaccharide-iron complexes might cause fewer stomach and gut side effects than iron salts.^[1] In pregnant women, ferrous bisglycinate, an amino-acid chelate, caused fewer of these side effects than other iron supplements.^[6]
• A dose every other day: in a 2025 review of 11 trials, side effects overall were about the same as with daily doses. A metallic taste was more common with daily doses.^[7]

Other effects

Supplements with 25 mg of iron or more can lower the absorption of zinc and its level in the blood.^[1] Case reports, some with 130 mg of iron, describe more serious harm to the stomach, such as gastritis.^[1]

In children in areas with malaria, a 2026 Cochrane review found that iron made little or no difference to clinical malaria overall.^[8] Where malaria prevention and treatment services were lacking, iron may raise malaria slightly.^[8]

Overdose

A single dose of more than 20 mg per kg of body weight can cause corrosive necrosis, the death of tissue in the gut. For a person who weighs 150 pounds, that is about 1,365 mg.^[1] A dose of about 60 mg per kg can cause organ failure, coma and death.^[1] From 1983 to 2000, at least 43 children in the US died after swallowing iron supplements.^[1] US labels warn that accidental overdose of iron products is a leading cause of fatal poisoning in children under 6.^[1]

Iron overload

Adults with normal gut function have very little risk of iron overload from food.^[1] People with hereditary hemochromatosis absorb too much iron. Without treatment, they usually show signs of iron toxicity by their 30s, such as liver cirrhosis and heart disease.^[1] About 1 in 10 White people carry the most common gene change for it, and about 4.4 in 1,000 White people have the disease.^[1]