Rifampin commonly interacts with oral contraceptives by causing what effect?

• A. Decreased effectiveness of oral contraceptives
• B. Increased effectiveness of oral contraceptives
• C. No interaction
• D. Decreased efficacy of erythromycin

Answer: (A)

Rifampin acts as a strong inducer of liver enzymes, especially CYP3A4. This speeds up the metabolism of the hormones in oral contraceptives (ethinyl estradiol and progestin), lowering their blood levels. With lower hormone levels, the suppression of ovulation is reduced, increasing the chance of breakthrough ovulation and unintended pregnancy. So the interaction decreases the effectiveness of oral contraceptives. A backup method of contraception during rifampin therapy and for about a month after stopping rifampin helps prevent pregnancy. The other options don’t fit because rifampin does not increase contraceptive effectiveness, does interact with oral contraceptives, and the question isn’t about erythromycin.