10. DRUG INTERACTIONS
Changes in contraceptive effectiveness associated with co-administration of other drugs:
a. Anti-infective agents and anticonvulsants
Contraceptive effectiveness may be reduced when hormonal contraceptives are co-administered with some antifungals, anticonvulsants, and other drugs that increase metabolism of contraceptive steroids. This could result in unintended pregnancy or breakthrough bleeding. Examples include barbiturates, griseofulvin, rifampin, phenylbutazone, phenytoin, carbamazepine, felbamate, oxcarbazepine, topiramate, and modafinil. Women may need to use an additional contraceptive method when taking such medications.
b. Anti-HIV protease inhibitors
Several of the anti-HIV protease inhibitors have been studied with co-administration of oral combination hormonal contraceptives; significant changes (increases and decreases) in the plasma levels of the estrogen and progestin have been noted in some cases. The efficacy and safety of hormonal contraceptive products may be affected with co-administration of anti-HIV protease inhibitors. Healthcare providers should refer to the label of the individual anti-HIV protease inhibitors for further drug-drug interaction information.
c. Herbal products
Herbal products containing St. John’s Wort (hypericum perforatum) may induce hepatic enzymes (cytochrome P450) and p-glycoprotein transporter and may reduce the effectiveness of contraceptive steroids. This may also result in breakthrough bleeding.