A woman taking combined oral contraceptive pills is prescribed rifampicin for TB treatment. She is counseled about potential contraceptive failure because rifampicin:

• A Inhibits CYP3A4, increasing estrogen and progestin plasma levels to supratherapeutic concentrations
• B Competes with ethinylestradiol for plasma protein binding, increasing free drug clearance
• C Blocks intestinal estrogen receptor reducing oral bioavailability
• D Induces CYP3A4, accelerating hepatic metabolism of ethinylestradiol and progestins below effective concentrations ✓

Explanation

Rifampicin is a potent inducer of cytochrome P450 enzymes, particularly CYP3A4, which is responsible for the first-pass and systemic metabolism of ethinylestradiol and most progestins in oral contraceptives. By markedly increasing CYP3A4 activity, rifampicin accelerates the breakdown of these hormones, reducing their plasma concentrations below effective contraceptive levels. Women on rifampicin should use barrier contraception or be switched to a non-OCP method.

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

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Written and medically reviewed by the StethoPrep medical team.