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Warfarin and metronidazole — GPhC CRA MCQ

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HardCardiovascular TherapeuticsWarfarin and metronidazoleGPhC CRA

A 77-year-old woman stable on warfarin for atrial fibrillation has an INR of 2.6 and is prescribed metronidazole 400 mg three times daily for five days. She has no bleeding. Which action is most appropriate before the first dose?

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Correct answer: D — Agree an interaction and early-INR plan with the anticoagulation service

Metronidazole can potentiate warfarin and substantially increase the INR. The antimicrobial indication and alternatives should be reviewed and, if co-administration is necessary, the anticoagulation service should define dosing and arrange additional INR monitoring, commonly after 3 to 5 days. A normal recent INR does not predict the interaction, and the pharmacist should not independently omit, halve or replace anticoagulation.

Reference: NHS SPS, Warfarin monitoring—interacting medicines: https://www.sps.nhs.uk/monitorings/warfarin-monitoring/