Although iron supplementation is commonly prescribed, the amount of elemental iron needed to achieve clinical efficacy, and the optimal method of supplementation, are under debate. Use of intravenous (IV) iron replacement is increasingly being advocated. We explore the physiology of iron supplementation, review clinical data suggesting that the typical oral dosing of iron may be excessive, and compare IV and oral methods of iron supplementation with a focus on inflammatory bowel disease (IBD). Both IV and oral iron can effectively raise hemoglobin levels in iron-deficiency anemia. There is no evidence that IV iron can raise hemoglobin at a faster pace. Side effects of oral iron are probably related to the relatively high doses of elemental iron that are typically prescribed. Emerging data suggest that low-dose iron has comparable efficacy, with fewer side effects. In IBD, both oral and IV iron are effective, and there is no convincing evidence that oral iron activates or exacerbates clinical symptoms. The use of a low starting dose of oral iron, such as one ferrous sulfate tablet per day, for treatment of iron deficiency is worth considering.