A 28-year-old woman on combined oral contraceptive pills becomes pregnant after starting rifampin for tuberculosis. The contraceptive failure is primarily due to rifampin's effect on which pharmacokinetic process?

• A Inhibition of CYP2D6, increasing estrogen levels
• B Induction of CYP3A4, accelerating estrogen metabolism ✓
• C Inhibition of P-glycoprotein, increasing estrogen absorption
• D Induction of Phase II conjugation, accelerating estrogen elimination

Explanation

Rifampin is a potent inducer of CYP3B4, the enzyme responsible for metabolizing ethinyl estradiol in oral contraceptives. This increases first-pass and systemic metabolism, reducing estrogen concentrations below contraceptive thresholds. CYP2D6 inhibition would increase, not decrease, drug levels. P-glycoprotein inhibition and Phase II induction are not the primary mechanisms of this interaction.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.