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Warfarin and antibiotic interaction — GPhC CRA MCQ

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ModerateDrug InteractionsWarfarin and antibiotic interactionGPhC CRA

A patient stabilised on warfarin is prescribed co-trimoxazole for a urinary infection. Her last INR was 2.4 and she has no bleeding symptoms. What is the most important interaction to flag?

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Correct answer: E — Markedly increased anticoagulant effect and bleeding risk

Co-trimoxazole can increase the anticoagulant effect of warfarin and significantly increase bleeding risk. The prescriber or anticoagulation clinic should be contacted to consider an alternative antibiotic or close INR management. The patient should be counselled to report bleeding or bruising. The interaction is not due to chelation.

Reference: BNF; NICE CKS