Cimetidine is a widely used antiulcer medication that is also a potent inhibitor of the mixed-function oxidase enzymes in the liver. This effect results in a number of clinically important interactions with other drugs, which are now being appreciated. Other effects of cimetidine, such as raising of gastric pH, alteration of liver blood flow, or alterations in renal secretory mechanisms, may also contribute to or result in clinically significant interactions. Current data document appreciable alterations in the metabolism or excretion of warfarin, theophylline, phenytoin, lidocaine, certain beta-adrenoceptor antagonists, certain benzodiazepines, and probably narcotic analgesics and procainamide. Thus, effects of these drugs and serum levels, where available, should be more closely followed when used in combination with cimetidine. Cimetidine appears to exacerbate the myelosuppressive effects of the nitrosoureas, and may also significantly alter the absorption of ketoconazole. Though other drugs may affect the absorption or elimination of cimetidine, the clinical implication of these effects is uncertain.