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Phenytoin and warfarin — GPhC CRA MCQ

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ModerateDrug InteractionsPhenytoin and warfarinGPhC CRA

A patient taking warfarin is started on phenytoin after a seizure. The anticoagulation clinic asks for pharmacy input. What is the most important interaction to flag?

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Correct answer: D — Unpredictable anticoagulant effect requiring close INR review

Phenytoin and warfarin interact through protein binding and enzyme effects. The best answer addresses the key safety, legal or pharmaceutical care issue in the vignette. The other options would either delay appropriate care, create a new risk, or fail to resolve the issue. This is consistent with the cited UK reference for safe pharmacy practice.

Reference: BNF