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AChEI-Anticholinergic Interaction — MRCPsych Paper B MCQ

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ModerateOld Age PsychiatryAChEI-Anticholinergic InteractionMRCPsych Paper B

A 75-year-old man with Alzheimer's disease is taking donepezil. He develops urinary urgency and urge incontinence, and his GP considers prescribing oral oxybutynin. What is the most important concern about combining these medicines?

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Correct answer: C — Antimuscarinic activity may reduce donepezil efficacy and worsen cognition

Oxybutynin is an antimuscarinic, whereas donepezil increases cholinergic transmission by inhibiting acetylcholinesterase. Their pharmacodynamic actions therefore oppose one another: oxybutynin may reduce the symptomatic efficacy of donepezil while adding central anticholinergic effects, including cognitive deterioration or confusion. This is particularly concerning in an older person with established dementia, and NICE recommends minimising medicines that increase anticholinergic burden where possible. Option A describes additive rather than opposing cholinergic effects. Oxybutynin does not clinically inhibit CYP3A4 as proposed in C. Donepezil does not reliably protect against oxybutynin's cognitive adverse effects, excluding D. Oxybutynin is not confined to the bladder and can produce central effects, so E is incorrect.

Reference: Electronic Medicines Compendium, Oxybutynin hydrochloride Aspire 5 mg prolonged-release tablets, SmPC sections 4.4 and 4.5, updated 20 January 2026: https://www.medicines.org.uk/emc/product/15798/smpc