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Warfarin–fluconazole interaction — PSA MCQ

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EasyPharmacologyWarfarin–fluconazole interactionPSAMRCGP AKTUKMLAPARA

A 71-year-old man takes warfarin for atrial fibrillation. His INR is stable at 2.5. He is prescribed a course of oral fluconazole for candidiasis. Which interaction and initial management are most appropriate?

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Correct answer: C — Enhanced anticoagulant effect and higher INR; arrange prompt INR monitoring and reduce warfarin if required

Fluconazole inhibits CYP2C9, reducing metabolism of the more active S-enantiomer of warfarin. This can enhance anticoagulation, increase the INR and raise bleeding risk; the UK SmPC reports prothrombin time prolonged up to twofold with bleeding events. Arrange an INR check promptly—NHS SPS advises testing 3–5 days after starting an interacting medicine—and adjust the warfarin dose according to the result; a reduction may be required. Fluconazole does not lower the INR, so increasing warfarin (B) is unsafe, and A underestimates a well-recognised interaction. The interaction alone does not justify permanently stopping warfarin for apixaban (D) or switching to dabigatran (E); any anticoagulant change needs a separate indication and a planned transition.

Reference: Electronic Medicines Compendium. Fluconazole 150 mg capsules: Summary of Product Characteristics, section 4.5 (Interactions — anticoagulants). https://www.medicines.org.uk/emc/product/4720/smpc