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Anticonvulsant-Benzodiazepine Interaction — MRCPsych Paper B MCQ

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HardSubstance MisuseAnticonvulsant-Benzodiazepine InteractionMRCPsych Paper B

A 46-year-old patient who has taken carbamazepine 400 mg twice daily for focal epilepsy for 8 weeks is admitted for medically assisted alcohol withdrawal. Liver function is normal, and a symptom-triggered oral diazepam regimen is selected. Compared with an otherwise similar patient not taking carbamazepine, which effect is most likely?

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Reveal the answer and explanation

Correct answer: A — Increased diazepam clearance and a shorter half-life, with reduced effect at a given dose

Carbamazepine is a potent hepatic enzyme inducer, including induction of CYP3A4. Diazepam undergoes oxidative metabolism through CYP3A4 and CYP2C19; established carbamazepine treatment can therefore increase diazepam clearance by up to approximately three-fold, shorten its half-life and reduce its effect at a given dose. Withdrawal treatment should be titrated against symptoms and sedation rather than automatically increasing the starting dose. Option B overlooks the pharmacokinetic interaction. Option E incorrectly describes carbamazepine as an inhibitor. Option C is incorrect because clearance of the parent drug is increased rather than unchanged. Option D is incorrect because altered renal elimination of diazepam conjugates is not the relevant mechanism.

Reference: Diazepam 10 mg Tablet, Summary of Product Characteristics, sections 4.5 and 5.2, updated 16 February 2026. https://www.medicines.org.uk/emc/product/101913/smpc