Why do people take iron?

Iron supplements are most often used for certain types of anemia. Anemia is a low level of red blood cells that can cause fatigue and other symptoms.  Anemia should never be considered a normal finding and must be evaluated by a medical doctor so it’s cause can be identified. No one should attempt to self-treat an anemic condition until they have been evaluated by a medical doctor to look for causes of the anemia.
There's good evidence that iron supplements can treat anemia caused by:
• pregnancy
• heavy menstrual periods
• kidney disease
• chemotherapy
While the evidence is weaker, iron supplements are also used for other types of anemia as well. Iron supplements are commonly recommended for infants and toddlers, teenage girls, and women who are pregnant or of childbearing age. Ask your doctor about other uses of iron supplements.
Iron supplements have also been studied for treatment of ADHD. While early data suggested a benefit, more study is needed before iron can be recommended for ADHD.