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AChEI-Anticholinergic Irrational — MRCPsych Paper B MCQ

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ModerateOld Age PsychiatryAChEI-Anticholinergic IrrationalMRCPsych Paper B

A 79-year-old woman with mild to moderate Alzheimer's disease has been cognitively stable while taking donepezil. Oxybutynin is started for urinary urgency. Over the next few weeks, her family notices increased confusion and worsening short-term memory. Examination and investigations identify no infection, metabolic disturbance or other precipitant of delirium. Which is the most likely pharmacological explanation?

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Correct answer: B — Oxybutynin antagonises cholinergic treatment and increases anticholinergic cognitive burden

Explanation lettering: D = shown as C · E = shown as D · C = shown as E

Oxybutynin is a muscarinic antagonist with clinically important central anticholinergic effects, particularly in older people. It can therefore worsen confusion and cognition while pharmacodynamically opposing donepezil, which increases cholinergic transmission through acetylcholinesterase inhibition. The oxybutynin SmPC specifically warns that co-administration may reduce cholinesterase-inhibitor efficacy, and NICE recommends minimising anticholinergic burden in dementia. Donepezil is not causing this through clinically relevant CYP3A4 inhibition (A). The drugs do not produce additive cholinergic stimulation (C). Oxybutynin is not restricted to peripheral effects (D), and the interaction is clinically relevant rather than merely theoretical (E). Co-prescription should prompt review of the indication and consideration of a less cognitively harmful alternative.

Reference: Electronic Medicines Compendium, Oxybutynin Hydrochloride Tablets 5 mg, Summary of Product Characteristics, sections 4.4 and 4.5, revised 21 April 2026. https://www.medicines.org.uk/emc/product/14310/smpc