Caution: Iron (ferrous sulphate) is the leading cause of accidental poisonings in children.^{68, 69, 70} The incidence of iron poisonings in young children increased dramatically in 1986. Many of these children obtained the iron from a child-resistant container opened by themselves or another child, or left open or improperly closed by an adult.⁷¹ Deaths in children have occurred from ingesting as little as 200 mg to as much as 5.85 grams of iron.⁷² Keep iron-containing supplements out of a child’s reach.

Haemochromatosis, haemosiderosis, polycythaemia, and iron-loading anaemias (such as thalassemia and sickle cell anaemia) are conditions involving excessive storage of iron. Supplementing iron can be quite dangerous for people with these diseases.

Supplemental amounts required to overcome iron deficiency can cause constipation. Sometimes switching the form of iron (see “Which forms of supplemental iron are best?” above), getting more exercise, or treating the constipation with fibre and fluids is helpful, though fibre can reduce iron absorption (see below). Sometimes the amount of iron must be reduced if constipation occurs.

Some researchers have linked excess iron levels to diabetes,⁷³cancer,⁷⁴ increased risk of infection,⁷⁵systemic lupus erythematosus (SLE),⁷⁶ exacerbation of rheumatoid arthritis,⁷⁷ and Huntington’s disease.⁷⁸ The greatest concern has surrounded the possibility that excess storage of iron in the body increases the risk of heart disease.^{79, 80, 81} Two analyses of published studies came to different conclusions about whether iron could increase heart disease risk.^{82, 83} One trial has suggested that such a link may exist, but only in some people (possibly smokers or those with elevated cholesterol levels).⁸⁴ The link between excess iron and any of the diseases mentioned earlier in this paragraph has not been definitively proven. Nonetheless, too much iron causes free radical damage, which can, in theory, promote or exacerbate most of these diseases. People who are not iron deficient should generally not take iron supplements.

Patients on kidney dialysis who are given injections of iron frequently experience “oxidative stress”. This is because iron is a pro-oxidant, meaning that it interacts with oxygen molecules in ways that can damage tissues. These adverse effects of iron therapy may be counteracted by supplementation with vitamin E.⁸⁵

Supplementation with iron, or iron and zinc, has been found to improve vitamin A status among children at high risk for deficiency of the three nutrients. ⁸⁶

People with hepatitis C who have failed to respond to interferon therapy have been found to have higher amounts of iron within the liver. Moreover, reduction of iron levels by drawing blood has been shown to decrease liver injury caused by hepatitis C.⁸⁷ Therefore, people with hepatitis C should avoid iron supplements.

In some people, particularly those with diabetes, metabolic syndrome, or liver disease, a genetic susceptibility to iron overload has been reported.⁸⁸