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Rifampicin-Warfarin Interaction — RACP Adult Medicine MCQ

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EasyClinical PharmacologyRifampicin-Warfarin InteractionRACP Adult Medicine

A 62-year-old man on warfarin for prosthetic mitral valve is commenced on rifampicin for latent TB treatment. His INR drops from 2.8 to 1.2 within 2 weeks. What is the mechanism?

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Correct answer: A — Rifampicin causes diarrhoea reducing warfarin absorption

Rifampicin is the most potent enzyme inducer used in clinical practice. It induces multiple CYP enzymes (particularly CYP2C9 and CYP3A4), dramatically accelerating warfarin metabolism and reducing its anticoagulant effect. The INR can fall within days to weeks. Warfarin doses often need to be doubled or tripled. Critically, when rifampicin is CEASED, enzyme induction reverses over 1–2 weeks, and the warfarin dose must be promptly reduced to avoid over-anticoagulation. This is one of the most dangerous drug interactions.

Reference: AMH – 2025 – Drug Interactions; eTG – 2025 – Antibiotic Guidelines