How does rifampin affect hormone-based therapies such as oral contraceptives?

• A. It induces hepatic enzymes, lowering drug levels and reducing efficacy of contraceptives and other hormonal therapies.
• B. It inhibits hepatic enzymes, increasing drug levels.
• C. It has no effect on hormonal therapies.
• D. It increases absorption of oral contraceptives.

Answer: (A)

Rifampin acts as a potent inducer of hepatic drug‑metabolizing enzymes, mainly the cytochrome P450 system. By speeding up the activity of enzymes like CYP3A4, it increases the metabolism and clearance of hormone components in oral contraceptives (estrogen and progestin). This lowers their plasma levels, which can reduce their contraceptive efficacy and raise the risk of breakthrough ovulation or unintended pregnancy. The same mechanism can affect other hormone-based therapies that rely on steady hormone levels.

Because of this enzyme induction, the usual approach is to use an alternative or backup form of contraception during rifampin therapy and for a period afterward, until enzyme activity returns toward baseline. The idea that rifampin inhibits enzymes or has no effect would predict higher hormone levels or no change, which is not observed with rifampin’s action. The key point is that induction of hepatic enzymes lowers hormone drug levels, compromising effectiveness.