St Johns Wort Warfarin — ORE Part 1 MCQ

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HardPharmacology & TherapeuticsSt Johns Wort WarfarinORE Part 1

A patient taking warfarin starts an over-the-counter preparation of St John's wort for low mood. Which change is most likely?

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Correct answer: A — Reduced anticoagulant effect with a lower INR

**A is correct.** St John's wort induces several drug-metabolising enzymes, including CYP2C9, and can reduce warfarin exposure. The expected consequence is reduced anticoagulant effect and a lower, potentially subtherapeutic INR, with thromboembolic risk. The UK SmPC lists concomitant warfarin use as contraindicated and specifically describes reduced anticoagulant effect. **B** and **E** reverse the interaction direction: they would be expected with reduced clearance or metabolic inhibition, not enzyme induction. **C** is wrong because this is a clinically significant interaction. **D** is wrong because the interaction is not confined to direct oral anticoagulants; warfarin is explicitly affected.

Reference: Thornton & Ross Ltd. Care St John's Wort Low Mood Relief Tablets: Summary of Product Characteristics, section 4.5 (Interactions), updated 2019. https://www.medicines.org.uk/emc/product/9038/smpc