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Ciprofloxacin Dose — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisCiprofloxacin DoseMRCPsych Paper B

A patient with treatment-resistant schizophrenia is stable on clozapine, with a recent trough plasma concentration within their established therapeutic range. Systemic oral ciprofloxacin is considered essential because suitable alternative antibacterials cannot be used. There are no current features of clozapine toxicity. What is the most appropriate management of the clozapine prescription when ciprofloxacin is started?

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Correct answer: A — Reduce the clozapine dose and arrange trough levels

The correct answer is A. Ciprofloxacin inhibits CYP1A2, an important pathway in clozapine metabolism, and can therefore raise clozapine and norclozapine concentrations. If systemic ciprofloxacin is unavoidable, the clozapine dose should be reduced, with close clinical surveillance for sedation, confusion, seizures and other toxicity, and trough plasma concentrations used to guide further adjustment. The precise reduction should be individualised rather than applying an unsupported fixed percentage. Maintaining, increasing or doubling the dose would expose the patient to greater toxicity. Routine abrupt discontinuation is also inappropriate because clozapine cessation can cause cholinergic rebound and rapid psychotic relapse; withholding may become necessary only if significant toxicity develops. The dose should be reviewed again when ciprofloxacin is stopped.

Reference: Electronic Medicines Compendium. Ciprofloxacin 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.4 and 4.5. Updated 9 June 2026. https://www.medicines.org.uk/emc/product/7256/smpc