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Start Smoking Dose — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisStart Smoking DoseMRCPsych Paper B

A 38-year-old man with treatment-resistant schizophrenia has remained clinically stable on clozapine while not smoking for the past 6 months. He now resumes smoking 15 manufactured cigarettes daily and intends to continue. What is the most appropriate approach to his clozapine dose?

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Correct answer: B — Measure a clozapine level and titrate the dose upwards if required

The correct answer is B. Polycyclic aromatic hydrocarbons in cigarette smoke induce CYP1A2, increasing clozapine metabolism and potentially lowering its plasma concentration and clinical efficacy. After sustained starting or resumption of smoking, a clozapine trough level should be obtained and the dose titrated under specialist supervision according to levels, mental state and tolerability. A routine increase of 50–100% is not recommended because the interaction varies substantially between patients. Dose reduction or halving would further increase the risk of subtherapeutic exposure. Leaving the dose unchanged without monitoring may permit relapse, while smoking is not an indication to discontinue clozapine.

Reference: NHS Specialist Pharmacy Service. Managing specific interactions with smoking, section 'Clozapine'. 2023. https://sps.nhs.uk/articles/managing-specific-interactions-with-smoking/