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Warfarin drug interaction — RACP Written MCQ

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ModerateClinical PharmacologyWarfarin drug interactionRACP Written

A 66-year-old man on warfarin for a mechanical mitral valve is prescribed trimethoprim-sulfamethoxazole for UTI. Four days later INR is 7.8 with epistaxis. What is the most likely cause?

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Correct answer: C — Antibiotic-warfarin interaction increasing anticoagulant effect

Trimethoprim-sulfamethoxazole can increase warfarin effect through metabolic and gut-flora mechanisms, causing supratherapeutic INR and bleeding.

Reference: AMH; Therapeutic Guidelines: Toxicology and drug-interaction principles