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Warfarin-antibiotic interaction — RCPSC IM MCQ

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ModerateHematologyWarfarin-antibiotic interactionRCPSC IM

A 77-year-old woman with atrial fibrillation takes warfarin. She develops melena after starting trimethoprim-sulfamethoxazole for UTI. INR is 6.8, hemoglobin has fallen by 28 g/L, and platelets are normal. What is the most likely cause?

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Correct answer: E — Warfarin potentiation by antibiotic interaction

Trimethoprim-sulfamethoxazole inhibits warfarin metabolism and can increase INR, predisposing to bleeding.

Reference: Thrombosis Canada warfarin clinical guidance