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St John's wort interaction — DFSRH MCQ

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HardPharmacology of hormonal contraceptionSt John's wort interactionDFSRH

A 28-year-old woman taking St John's wort for low mood uses a combined oral contraceptive pill and has breakthrough bleeding. She had UPSI 2 days ago during week 2. What is the most appropriate management?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Advise St John's wort can reduce hormonal contraceptive effectiveness and assess need for EC

St John's wort is an enzyme inducer and can reduce effectiveness of hormonal contraception, so EC assessment and alternative contraception counselling are needed. Herbal origin does not mean interaction-free. Increasing the pill dose or using the patch does not reliably solve enzyme induction, and UPA followed by immediate CHC can reduce EC effectiveness. The pearl is to ask specifically about over-the-counter and herbal products.

Reference: FSRH Drug Interactions with Hormonal Contraception; BNF