Can you get too much iron?

The Tolerable Upper Intake Level for iron is 45 mg a day for adults.

What this means for you

Iron is the one where the safety question is not academic, so this page is blunter than the others.

Not for everyone. Accidental overdose of iron products is a leading cause of fatal poisoning in children under six. The tablets look like sweets. A single intake above 20 mg per kg of body weight, roughly 1,365 mg for a 150-pound adult and far less for a toddler, can burn the lining of the gut and cause shock and organ failure. If there is iron in your house and a small child in it, the bottle belongs somewhere they cannot reach, not in a handbag or a bedside drawer.

At ordinary doses the unpleasantness is milder and common. Iron supplements cause stomach upset, constipation, nausea, abdominal pain and diarrhoea in a lot of people, which is the usual reason they get abandoned. If that is happening to you, it is a conversation to have with whoever prescribed the tablets rather than a reason to quietly stop.

Established. Ferrous sulfate, the standard tablet, roughly doubles the odds of stomach trouble against placebo and triples them against the same iron given by drip. That comes from 43 trials pooled together, so it is the expected experience rather than bad luck.

The obvious next move is to ask for a smaller dose. The trials do not back it. Across those 43 trials there was no relationship between the dose of iron and the odds of side effects. The doses studied sat in a narrow treatment range, so a much smaller one might still be gentler. What the data do rule out is the assumption that halving your tablet halves the trouble.

The upper limit is 45 mg a day for adults, and food does not come close to it. This is about tablets.

Established. Some people are in danger from ordinary amounts. In hereditary haemochromatosis the body keeps absorbing iron it does not need, and untreated overload typically causes liver cirrhosis and heart disease by a person's thirties. This is why the advice to take iron for tiredness without a blood test is bad advice in both directions.

The same warning runs the other way round. A low haemoglobin reading is a reason to find out why, and not yet a reason to start taking iron. Among 310 young children in one Peruvian study, iron deficiency explained 27.5 per cent of the anaemia and inflammation explained 45.9 per cent. In 2024 WHO also moved the haemoglobin cutoff for children aged 6 to 23 months, which took the share counted as anaemic in that same sample from 62 to 47 per cent with nothing about the children changing. Iron given for an anaemia that was never about iron is all dose and no benefit.

Not for everyone. One harm signal depends on where you live rather than on how much you take. In places where malaria is common and prevention and treatment are not available, children given iron had about 15 per cent more clinical malaria. Where those services did exist, the same supplements slightly lowered it. This comes from comparing subgroups after the fact rather than from a trial built to test it, so what it argues for is iron programmes in those settings arriving alongside malaria care. Withholding iron is not what the reviewers concluded.

Myth. The larger fear attached to that finding does not survive the numbers. Across 40 trials, deaths among children given iron in malaria areas were no different from deaths without it, and hospital visits were no different either. Severe malaria fell slightly, on high-certainty evidence. The mortality estimate is imprecise enough to hide a real increase, so treat this as reassurance and not as a clean bill of health.

One practical detail. Iron reduces the effect of levothyroxine, and the label says to keep the two at least four hours apart.

One place the balance is argued out properly is pregnancy, because that is where iron is given routinely to healthy people. Across 12 trials in 2,423 women, 21.6 per cent on iron reported side effects against 18.0 per cent without, and the reviewers rated that evidence very uncertain. The same review found daily iron cut anaemia from 7.4 per cent to 4.0 per cent. A clear benefit and an unmeasured cost is a fair summary of where routine iron sits, and it is the reason a test beats a guess. The full review is here.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Iron: why the number on the label is not the number your body gets