A woman taking a combined oral contraceptive pill (COCP) is prescribed rifampicin for tuberculosis. Rifampicin reduces OCP efficacy primarily by:

• A Binding directly to estrogen receptors, acting as a competitive antagonist
• B Inhibiting enterohepatic recirculation of conjugated estrogens by killing intestinal flora
• C Increasing SHBG levels, reducing free ethinylestradiol concentrations
• D Inducing CYP3A4 and increasing intestinal P-glycoprotein expression, accelerating metabolism and reducing plasma levels of ethinylestradiol and progestin ✓

Explanation

Rifampicin is one of the most potent inducers of hepatic CYP3A4 and intestinal P-glycoprotein. This accelerates the metabolism of ethinylestradiol and progestins in OCP, reducing their plasma concentrations to sub-therapeutic levels and impairing ovulation suppression. Women on rifampicin require additional contraception (barrier methods) throughout treatment and for 4 weeks after stopping rifampicin.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.