The literature on cimetidine drug interactions has been thoroughly reviewed. Several different mechanisms have been proposed for cimetidine-related drug interactions. These mechanisms include: (1) impaired hepatic drug metabolism due to inhibition of hepatic microsomal enzymes, (2) reduced hepatic blood flow, resulting in decreased clearance of drugs that are highly extracted by the liver, (3) increased potential for myelosuppression when administered concurrently with other drugs capable of causing myelosuppression, and (4) altered bioavailability of acid-labile drugs. Cimetidine binds reversibly to the hepatic cytochrome P-450 and P-448 systems, resulting in decreased metabolism of drugs that undergo Phase I reactions (e.g., dealkylation and hydroxylation). In contrast, glucuronidation pathways are unaffected. The rapid onset and reversal of cimetidine's inhibition of hepatic metabolism indicates an effect on hepatic enzyme systems. Cimetidine also has been reported to decrease hepatic blood flow. Drugs that are highly extracted by the liver, such as propranolol, lidocaine, and morphine, may be postulated to have a decreased hepatic clearance. Cimetidine, through its effect on gastric pH, may increase the absorption of acid-labile drugs or may decrease the absorption of drugs. There have been reports of increased potential for myelosuppression when cimetidine is administered concurrently with drugs capable of causing bone marrow suppression. An understanding of the mechanisms involved in cimetidine drug interactions allows the clinician to prevent and predict these interactions.