The clinical importance of drug interactions with antiulcer therapy. The overall safety of a given drug is determined by its toxicity, side effects, and drug-drug interactions. Thus, a clarification of the mechanisms, importance, and clinical implications of any drug-drug interaction with antiulcer therapy is critical to the use of antiulcer medications. Drug-drug interactions may occur as a result of changes in absorption, metabolism, distribution, or excretion. Fortunately, drug distribution or protein binding is unchanged by antiulcer therapy. Antiulcer drugs may affect absorption by several mechanisms. Ionized medications may bind to the divalent cations of antacids and sucralfate to result in poorly absorbed complexes. Reduced gastric acid may decrease the absorption of medications that are weak bases while enhancing the absorption of weak acids. Drug absorption may be impaired by delayed gastric emptying. Several H2-receptor antagonists, including cimetidine and to a lesser extent ranitidine, and the proton pump inhibitor, omeprazole, may reduce the hepatic degradation of drugs metabolized by the cytochrome P450 system. The degree to which such agents alter drug metabolism is determined by the patient's age, genetics, duration of therapy, degree of cytochrome P450 binding, and the regimen. Because the clinical importance of this interaction cannot always be predicted, caution is recommended whenever drugs metabolized by this system are used concurrently. Development of an understanding of the ways in which drug metabolism interactions occur may lead to more effective and safe use of these medications. 