carbamazepine and ethinylestradiol/levonorgestrel

Carbamazepine induces hepatic cytochrome P450 enzymes, particularly CYP3A4, and also increases sex hormone-binding globulin (SHBG). This accelerates metabolism of ethinylestradiol and levonorgestrel, lowering their plasma levels, while raised SHBG binds more progestin, further reducing free hormone concentrations.

Contraceptive efficacy is reduced, risking unintended pregnancy. Breakthrough bleeding may occur. There is also potential for hormonal imbalance and altered menstrual cycles. The effect can persist for some weeks after carbamazepine is stopped.

Avoid co-prescribing carbamazepine and combined oral contraceptives. If concurrent use is necessary, use an alternative contraceptive method such as a copper intrauterine device or a barrier method. Consider a progestogen-only method not reliant on CYP3A4, or a higher-dose combined pill, but no method is fully reliable. Counsel on breakthrough bleeding and the need for additional contraception during and for at least 28 days after stopping carbamazepine. Monitor for pregnancy.

Well-established. Multiple clinical studies and case reports confirm reduced contraceptive effectiveness with enzyme-inducing antiepileptics. Guideline recommendations support the use of additional contraception.