A 28-year-old woman on combined oral contraceptive pills (ethinyl estradiol 30 mcg + desogestrel 150 mcg) is diagnosed with pulmonary tuberculosis and started on rifampicin, isoniazid, pyrazinamide, and ethambutol. She is counseled that her OCP may fail. The specific pharmacokinetic mechanism by which rifampicin reduces contraceptive efficacy is:

• A Rifampicin inhibits CYP3A4, increasing estrogen levels
• B Rifampicin increases renal clearance of ethinyl estradiol
• C Rifampicin causes malabsorption of oral contraceptives in the gut
• D Rifampicin induces CYP3A4, accelerating estrogen and progestin metabolism ✓

Explanation

Rifampicin is a potent inducer of hepatic CYP3A4, the enzyme responsible for metabolizing both ethinyl estradiol and progestins. This increases their clearance, reducing serum concentrations below the threshold needed for ovulation suppression. The interaction is clinically significant and requires additional contraceptive measures. Option A is the opposite of the true mechanism and is a common distractor.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.