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Warfarin and clarithromycin — GPhC CRA MCQ

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HardDrug InteractionsWarfarin and clarithromycinGPhC CRA

A 69-year-old woman with AF takes warfarin and has an INR of 4.8 after starting clarithromycin for pneumonia. She reports bruising but no major bleeding. The GP asks whether the antibiotic could be relevant. What is the most important interaction to flag?

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Correct answer: B — Clarithromycin can increase warfarin effect and INR, requiring urgent INR management

Macrolides such as clarithromycin can increase the anticoagulant effect of warfarin through metabolic inhibition and illness-related factors may also raise INR. An INR of 4.8 with bruising needs prompt clinical management according to anticoagulation protocols. The interaction is not reduced warfarin effect. Absorption failure and hyperglycaemia are not the principal concerns.

Reference: BNF