A 45-year-old patient on rifampicin for tuberculosis notices his oral contraceptive pills are failing. The most likely explanation is:

• A Rifampicin induces CYP3A4 and P-glycoprotein, accelerating estrogen and progesterone metabolism ✓
• B Rifampicin inhibits CYP3A4, reducing estrogen metabolism and causing receptor downregulation
• C Rifampicin reduces enterohepatic circulation of conjugated estrogens by killing gut flora
• D Rifampicin competes with estrogen for hepatic protein binding, increasing free drug clearance

Explanation

Rifampicin is one of the most potent inducers of hepatic cytochrome P450 enzymes (especially CYP3A4, CYP2C9) and P-glycoprotein. This accelerates the metabolism of oral contraceptive steroids (estrogen and progestins), dramatically reducing their plasma levels and contraceptive efficacy. Women on rifampicin should be counseled to use additional barrier contraceptive methods. This same induction underlies rifampicin's interactions with warfarin, antiretrovirals, azole antifungals, and methadone.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.