Quick answer

Stomach and bowel complaints are the most common side effect of iron supplements: in 20 placebo-controlled trials, ferrous sulfate more than doubled their odds. Taking iron with food or every other day can make them less likely. Adults should stay under 45 mg a day unless a doctor advises more.^1,2,3

Key facts

Doses of 45 mg a day or more are the most likely to upset the stomach.²
Iron supplements with 25 mg or more can lower zinc absorption.²
US rules require a child-safety warning on the label of every iron tablet and capsule.²
Containers holding 250 mg of iron or more must be child-resistant.²

The common side effects

In 20 placebo-controlled trials with 3,168 adults, ferrous sulfate more than doubled the odds of stomach and bowel side effects (odds ratio 2.32), and the analysis found no clear link with dose.¹

In trials in women who menstruate, iron roughly doubled reports of constipation and of loose stools.⁴

The usual complaints are an upset stomach, nausea, cramping, constipation and loose stools. NIH links them to high doses, especially on an empty stomach, although the pooled trials of ferrous sulfate found no clear pattern by dose.^1,5

Ways to make side effects less likely

In a trial of 150 young women with low iron stores, iron taken on alternate days caused fewer stomach and bowel side effects than the same total amount taken daily, and left fewer women with low iron after 6 months.³

Heme iron polypeptides, carbonyl iron, iron amino acid chelates and polysaccharide iron complexes might cause fewer stomach and bowel side effects than ferrous or ferric salts.²

Taking iron with food also lessens stomach upset, although some foods and drinks, coffee most of all, cut how much is absorbed. If one product disagrees with you, a different form or a smaller amount is worth raising with your clinician.^2,6

Serious harm at high doses

Case reports, some involving 130 mg of iron, describe gastritis and damage to the stomach lining.²

A single intake of more than 20 mg of iron per kg of body weight, about 1,365 mg for a 150 pound adult, can damage the gut and lead to shock and organ failure. Around 60 mg per kg can cause coma, seizures and death.²

These amounts are far above any normal supplement dose for an adult, but not for a small child. For a toddler weighing 15 kg, 20 mg per kg is 300 mg, the iron in five 65 mg tablets.²

Keep iron away from children

Between 1983 and 2000, at least 43 children in the US died after swallowing iron supplements, and from 1983 to 1991 iron caused about a third of the poisoning deaths reported in children. After FDA required single-dose packaging and strong warnings in 1997, only one such death was reported from 1998 to 2002.²

Today iron tablets and capsules must say that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Store them out of reach, and call a doctor or poison control center straight away if a child swallows any.^2,5

Who should avoid iron or ask first

Hereditary hemochromatosis makes the body absorb too much iron. About 1 in 10 White people carry the most common gene change and about 4.4 in 1,000 have the condition; without care, iron reaches toxic levels, often by the 30s. People with it should avoid iron and vitamin C supplements.²

Extra iron does not fix every kind of anemia. In anemia linked to long-term illness, the body moves iron into storage rather than using it, and more iron from food or supplements usually does not help.⁵

In pregnancy the need for iron rises, and the American College of Obstetricians and Gynecologists recommends low-dose iron from the first trimester. Ask your prenatal provider which product and amount to use.²

Ask first if you have kidney disease or another long-term illness
Ask first before giving any iron product to a child
Ask first if you have had stomach or bowel surgery or have a bowel condition²

Medicines iron can affect

Iron can bind some medicines in the gut. NIH and the drug labels name levothyroxine, levodopa, tetracycline and quinolone antibiotics, and dolutegravir. Medicines that lower stomach acid work the other way, reducing how much iron is absorbed. A pharmacist can set a schedule that keeps them from clashing.^5,7,8,9

Questions people ask

Why does iron upset my stomach?

Iron salts can irritate the gut, more so in large doses and on an empty stomach. Taking iron with food, trying another form or taking it every other day may help. Ask your clinician before changing an amount you were prescribed.^2,3

Is it normal for iron to cause constipation?

It is common. In trials in women, iron made hard stools about twice as frequent, and across 20 placebo-controlled trials ferrous sulfate more than doubled the odds of stomach and bowel side effects. Tell your clinician if it is severe or does not settle.^1,4

Can iron supplements hurt a toddler?

Yes. A handful of adult tablets can be a dangerous dose for a small child, and accidental iron overdose is a leading cause of fatal poisoning in children under 6. Call a doctor or poison control center right away if a child swallows iron.^2,5

Is iron safe for people with kidney disease?

Iron is sometimes part of care for kidney disease, but the amount, and whether it is given by mouth or by vein, are decisions for the kidney care team. Do not start iron on your own.²

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