Iron supplements, also known as iron salts and iron pills, are a number of iron formulations used to treat and prevent iron deficiency including iron deficiency anemia. For prevention they are only recommended in those with poor absorption, heavy menstrual periods, pregnancy, hemodialysis, or a diet low in iron. Prevention may also be used in low birth weight babies. They are taken by mouth, injection into a vein, or injection into a muscle. While benefits may be seen in days, up to two months may be required until iron levels return to normal.

Common side effects include constipation, abdominal pain, dark stools, and diarrhea. Other side effects, which may occur with excessive use, include iron overload and iron toxicity. Ferrous salts used as supplements by mouth include ferrous fumarate, ferrous gluconate, ferrous succinate, and ferrous sulfate. Injectable forms include iron dextran and iron sucrose. They work by providing the iron needed for making red blood cells.

Iron pills have been used medically since at least 1681, with an easy-to-use formulation being created in 1832. Ferrous salt is on the World Health Organization's List of Essential Medicines. Ferrous salts are available as a generic medication and over the counter. Slow release formulations, while available, are not recommended. In 2020, ferrous sulfate was the 116th most commonly prescribed medication in the United States, with more than 5 million prescriptions.