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Warfarin and metronidazole — GPhC CRA MCQ

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HardDrug InteractionsWarfarin and metronidazoleGPhC CRA

A 65-year-old man on warfarin for a mechanical valve presents a prescription for metronidazole 400mg three times daily for dental infection. His INR was 2.8 yesterday. He has had a previous nosebleed after antibiotics. What is the most important interaction to flag?

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Correct answer: B — Metronidazole may increase anticoagulant effect and bleeding risk

Metronidazole can increase the anticoagulant effect of warfarin and raise bleeding risk. This is high stakes in a mechanical-valve patient because both bleeding and thrombosis risks require careful management. The anticoagulation clinic or prescriber should be contacted for INR monitoring and dose advice. The other options do not reflect the clinically important interaction.

Reference: BNF Warfarin interactions; BNF Metronidazole