A woman on combined oral contraceptive pills (COCPs) is started on rifampicin for TB treatment. Her GP advises extra contraceptive precautions. The pharmacokinetic basis for this interaction is:

• A Rifampicin inhibits CYP3A4, leading to elevated ethinylestradiol levels causing toxicity
• B Rifampicin causes malabsorption of OCP hormones by damaging intestinal enterocytes
• C Rifampicin is a potent inducer of CYP3A4 and P-glycoprotein, increasing metabolism and reducing plasma levels of ethinylestradiol and progestogens ✓
• D Rifampicin competitively displaces ethinylestradiol from sex hormone-binding globulin (SHBG)

Explanation

Rifampicin is among the most potent inducers of CYP3A4 and P-glycoprotein known in clinical practice. Ethinylestradiol and progestogens in COCPs are substrates of CYP3A4; their metabolism is dramatically increased, reducing plasma hormone levels below effective contraceptive concentrations. This interaction leads to breakthrough ovulation and contraceptive failure. Barrier contraception or higher-dose pills should be used during rifampicin therapy and for 4–8 weeks after stopping it.

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

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Written and medically reviewed by the StethoPrep medical team.