Safe Antibiotic Clozapine — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisSafe Antibiotic ClozapineMRCPsych Paper B

A patient whose schizophrenia is stable on clozapine requires antibacterial treatment. Assuming that each option is microbiologically appropriate and that there is no allergy or other contraindication, which antibacterial has the least potential to alter the plasma clozapine concentration through cytochrome P450 inhibition or induction?

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Correct answer: B — Amoxicillin

Amoxicillin has the least potential for a direct pharmacokinetic interaction with clozapine because it is not a clinically relevant CYP inhibitor or inducer. Ciprofloxacin inhibits CYP1A2 and can substantially increase clozapine exposure, increasing dose-related toxicity such as sedation, seizures and gastrointestinal hypomotility. Erythromycin and clarithromycin may inhibit clozapine metabolism; macrolides can also add to QT-prolongation risk. Rifampicin is a potent enzyme inducer that can markedly reduce clozapine concentrations and precipitate loss of antipsychotic efficacy. The antibacterial must still be selected according to the infection, susceptibility, allergy history and local antimicrobial guidance. Additionally, infection itself—particularly pneumonia or another serious inflammatory illness—can inhibit clozapine metabolism, so toxicity assessment and clozapine-level monitoring may be required even when amoxicillin is used.

Reference: electronic Medicines Compendium, Clozapine Viatris 100 mg Tablets SmPC, section 4.5, updated 13 March 2026, https://www.medicines.org.uk/emc/product/15498/smpc; and Amoxicillin 500 mg Capsules SmPC, section 4.5, updated 2 March 2026, https://www.medicines.org.uk/emc/product/10638/smpc