Rifampin, an antituberculosis agent, is usually administered for nine to 12 months with other antituberculosis drugs or drugs from other classes. A potential for drug interactions often exists because this drug is a potent inducer of drug metabolism. Rifampin causes a proliferation of the smooth endoplasmic reticulum and an increase in the cytochrome P-450 content in the liver. Rifampin's enzyme-induction effect is selective; therefore, it is not always clear which agents will be affected. Studies and case reports have demonstrated that rifampin accelerates the metabolism of several drugs, including oral anticoagulants, oral contraceptives, glucocorticoids, digitoxin, quinidine, methadone, hypoglycemics, and barbiturates. Rifampin interacts with other agents, but further study is needed to demonstrate the clinical importance of these interactions. In addition to inducing drug-metabolizing enzymes, rifampin may cause alterations in absorption and hepatic uptake. Future investigations will probably identify new rifampin drug interactions.