Rifampin is a microsomal enzyme inducer-increases several CYP450 isoenzymes, including CYP3A4, CYP2D6, CYP1A2 and CYP2C subfamily. It thus enhances its own metabolism as well as that of many drugs including warfarin, oral contraceptives, coicosteroids, sulfonylureas, digitoxin, steroids, HIVprotease inhibitors, non-nucleoside reverse transcriptase inhibitors (NNis), theophylline, metoprolol, fluconazole, ketoconazole, etc. Contraceptive failures have occurred: switch over to an oral contraceptive containing higher dose (50 pg) of estrogen or use alternative method of contraception. (Essentials of medical pharmacology, K.D Tripathi, page 741)