H2-blocking agents, such as cimetidine or ranitidine are used in numerous patients. This treatment is often associated with the co-administration of a variety of other drugs. From clinical observations and pharmacokinetic studies it is obvious that even short-term treatment with therapeutic doses of cimetidine inhibits the hepatic elimination of antipyrine, warfarin, diazepam, desmethyldiazepam, chlordiazepoxide, propranolol, labetalol, metoprolol, phenytoin, carbamazepine, chlormethiazole, theophylline and caffeine. All these drugs are metabolized by cytochrome-dependent so-called phase I reactions. Cimetidine can interact with drug binding to the cytochrome P450 system leading to impaired drug metabolism. On the other hand drugs which are eliminated by glucuronidation (cytochrome independent phase II reaction), such as oxazepam and lorazepam are not affected by cimetidine. Other H2-blocking agents (ranitidine, oxmetidine) did not impair the elimination of antipyrine, warfarin, diazepam or propranolol. Furthermore, cimetidine and ranitidine might slightly reduce hepatic blood flow which could reduce the elimination of drugs with high hepatic clearance.