Safe Antibiotic with Clozapine — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisSafe Antibiotic with ClozapineMRCPsych Paper B

A patient with treatment-resistant schizophrenia is stable on clozapine and develops community-acquired pneumonia. Culture and susceptibility results indicate that each of the following antibacterial agents would be microbiologically active. Which has the lowest potential for a clinically significant pharmacokinetic or haematological interaction with clozapine?

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Correct answer: C — Amoxicillin

Amoxicillin has the lowest potential for a clinically significant interaction with clozapine. It is not a clinically important CYP1A2 inhibitor or inducer, and clozapine is not included among the relevant interactions in its SmPC. Ciprofloxacin inhibits CYP1A2 and can substantially increase clozapine exposure and toxicity. Rifampicin induces CYP enzymes and may markedly reduce clozapine concentrations, risking relapse. Co-trimoxazole is a sulfonamide with bone-marrow-suppressing potential and should be avoided with clozapine where possible. Erythromycin inhibits CYP3A4; the clozapine SmPC describes major pharmacokinetic interaction as generally unlikely but notes reported cases, making it less suitable than amoxicillin. Importantly, pneumonia and associated smoking cessation can themselves increase clozapine levels, so toxicity assessment and plasma-level monitoring may still be required regardless of antibiotic choice.

Reference: electronic Medicines Compendium. Clozaril 200 mg Tablets/Clozapine Viatris 200 mg Tablets, SmPC sections 4.4–4.5, updated 20 April 2026, https://www.medicines.org.uk/emc/product/102057/smpc; Amoxicillin 500 mg Capsules, SmPC sections 4.1 and 4.5, updated 2 March 2026, https://www.medicines.org.uk/emc/product/10638/smpc