A patient on rifampicin-based anti-TB therapy is started on an oral contraceptive. The pharmacokinetic interaction most likely to occur is:
- A Rifampicin is a potent CYP3A4 and P-glycoprotein inducer, markedly reducing plasma levels of ethinylestradiol and progestins, risking contraceptive failure ✓
- B Rifampicin inhibits CYP3A4, increasing ethinylestradiol levels and causing oestrogen toxicity
- C Ethinylestradiol competitively inhibits rifampicin's hepatic metabolism, causing rifampicin toxicity
- D Rifampicin inhibits intestinal P-glycoprotein, increasing oral bioavailability of both estrogen and progestin
Correct answer: A. Rifampicin is a potent CYP3A4 and P-glycoprotein inducer, markedly reducing plasma levels of ethinylestradiol and progestins, risking contraceptive failure
Explanation
Rifampicin is one of the most potent inducers of CYP1A2, CYP2C9, CYP2C19, CYP3A4, and P-glycoprotein. It dramatically increases first-pass and systemic metabolism of ethinylestradiol and progestins, reducing their plasma concentrations below effective contraceptive levels. Women on rifampicin require additional non-hormonal contraception or higher-dose formulations throughout and for at least 4 weeks after treatment.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.