When prescribing oral iron preparations, consideration must be given in particular to the question of bioavailability. In this connection, ferrous sulfate preparations are to be preferred. The simultaneous ingestion of tea, coffee or dairy products reduces the absorption of iron, while ascorbic acid increases it. Gastrointestinal side effects of oral iron substitution are usually dose-dependent. If absorption or mobilization disorders, gastrointestinal diseases or signs of intolerance to oral administration are present, iron must be administered via the parenteral route. To avoid toxicity, accurate calculation of the dose is necessary before injection. Since intolerance reactions extending to anaphylactic shock can occur, the indication for iron preparations must be rigorously applied.