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St John's wort and warfarin — GPhC CRA MCQ

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HardDrug InteractionsSt John's wort and warfarinGPhC CRA

A patient prescribed warfarin for a mechanical valve starts a new herbal product bought online for low mood. The label lists St John's wort. His INR has fallen from 2.8 to 1.7 over 2 weeks. What is the most important interaction to flag?

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Correct answer: A — St John's wort can reduce warfarin effect through enzyme induction

St John's wort is an enzyme inducer and can reduce the effect of warfarin, lowering INR and increasing thrombotic risk. This is particularly serious in a patient with a mechanical valve. The patient should be advised to stop the herbal product and contact the anticoagulation service urgently. The observed INR fall fits reduced anticoagulant effect rather than increased bleeding risk.

Reference: BNF