Iron supplementation may cause constipation. Clinical iron supplementation is mainly used in the treatment of iron deficiency anemia and in the treatment of iron deficiency in some infants, children, adolescents, and women. Moderate dietary iron supplementation does not usually cause constipation, but oral or intravenous iron supplementation may cause constipation. Oral iron supplements are mainly absorbed through the intestines, which will absorb water in the intestines, resulting in dry stools that are not easily discharged, causing constipation. Iron may also irritate the gastrointestinal tract, slowing down bowel movements, which may also cause constipation. Unabsorbed iron is excreted in the feces in the form of black stools. Therefore, it is recommended to take vitamin C and other medications to promote the absorption of iron during iron supplementation, and to drink plenty of water and exercise moderately to prevent or relieve constipation. Communicate with your doctor promptly when adverse reactions occur to avoid serious consequences.