A 30-year-old woman on oral contraceptives is prescribed rifampicin for a latent TB infection contact. She is counselled to use additional contraception. The pharmacokinetic basis of this drug interaction is:
- A Rifampicin inhibits CYP3A4, increasing ethinyl estradiol plasma levels to toxic concentrations
- B Rifampicin chelates estrogen in the GIT, reducing its absorption
- C Rifampicin induces hepatic CYP enzymes and P-glycoprotein, markedly increasing OCP metabolism ✓
- D Rifampicin competes with estrogen for plasma protein binding, increasing free estrogen clearance
Correct answer: C. Rifampicin induces hepatic CYP enzymes and P-glycoprotein, markedly increasing OCP metabolism
Explanation
Rifampicin is one of the most potent inducers of hepatic cytochrome P450 enzymes (CYP3A4, CYP2C9) and P-glycoprotein. This leads to accelerated first-pass and systemic metabolism of ethinyl estradiol and progestins in oral contraceptives, reducing plasma concentrations below effective levels and increasing the risk of contraceptive failure and unintended pregnancy. This interaction also affects antiretrovirals, warfarin, and corticosteroids.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.