Drug interactions and hormonal contraception

There are several classes/types of drugs that interact with hormonal contraception, which can increase or reduce the effectiveness of the contraception. Often, there are no studies that directly inform how a drug may be affected by exposure to hormonal contraception. Information is usually extrapolated from studies of interactions between other drugs, which are then applied to hormonal contraception. Sometimes, individual case studies are also used. Faculty of Sexual and Reproductive Health (FSRH) guidance (2022) errs on the side of caution where there is potential to reduce the effectiveness of a hormonal contraception, and this is even more important when a teratogenic drug is in use.

The types of drug interactions that affect hormonal contraception are related to the induction or inhibition of hepatic cytochrome P450 enzymes (CYP450), so a basic understanding of the enzymes is necessary. Drug interactions occur when a drug changes the bioavailability of another drug by altering the absorption, distribution, metabolism or excretion. Cytochrome P450 (CYP450) is a haemoprotein that plays a key role in the metabolism of drugs and other synthetic chemicals (McDonnell and Dang, 2013).