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Drugs affecting hepatic microsomal enzyme: Clinically important drug interactions may occur when RITA is administered with other drugs metabolized by the hepatic microsomal cytochrome P-450 (CYP) enzyme system. Metabolism of estrogens is mediated by the CYP3A4 isoenzyme, and the possibility exists that drugs that induce this isoenzyme may reduce ethinyl estradiol concentrations. Rifampin reportedly decreases contraceptive efficacy and increases breakthrough bleeding during concomitant use with oral contraceptives. These effects have been attributed to enhanced metabolism of both the estrogenic and progestinic components of oral contraceptives, presumably by induction of hepatic microsomal enzymes. It has been suggested that similar effects may occur during concomitant therapy with other known inducers of hepatic microsomal enzymes, including barbiturates, bosentan, carbamazepine, dexamethasone, griseofulvin, phenytoin, felbamate, oxcarbazepine, rifabutin, modafinil, topiramate and primidone. Herbal supplements containing St. John's wort may decrease contraceptive efficacy of estrogen-progestin contraceptives. Because of the risk of contraceptive failure during concomitant use of estrogen-progestin contraceptives with known inducers of hepatic microsomal enzymes, it has been suggested that alternate methods of contraceptive preparations with increased dosage be considered.
Anti-retroviral agents: Concomitant use of oral contraceptives and some HIV-protease inhibitors or non-nucleoside reverse transcriptase inhibitors may reduce the efficacy of the oral contraceptive.
