Iron supplements: a common cause of drug interactions.

Iron-drug interactions of clinical significance may occur in many patients and involve a large number of therapies. Concurrent ingestion of iron causes marked decreases in the bioavailability of a number of drugs. The affected drugs, tetracycline, tetracycline derivatives (doxycycline, methacycline and oxytetracycline), penicillamine, methyldopa, levodopa, carbidopa and ciprofloxacin have diverse chemical structures and clinical effects. The major mechanism of these drug interactions is the formation of iron-drug complexes (chelation or binding of iron by the involved drug). A large number of other important and commonly used drugs such as thyroxine, captopril and folic acid have been demonstrated to form stable complexes with iron. There is little known about the effects of concurrent therapy with iron supplements for most of the drugs.