Tobacco smoke induces the liver enzyme CYP1A2, which clears clozapine. When a patient stops smoking, or moves into a smoke-free ward, that induction wears off over about a week and clozapine levels can rise substantially on an unchanged dose. The reverse happens when a patient starts smoking again after discharge.
Nicotine replacement does not induce the enzyme, because the effect comes from hydrocarbons in smoke, not from nicotine. Ask about smoking at every review, warn patients and carers about drowsiness, hypersalivation, confusion or seizures after quitting, and measure a level and consider a dose reduction when smoking status changes.
- Ask about smoking status at every clozapine review and on admission.
- Expect levels to rise within about a week of stopping smoking.
- Nicotine replacement does not prevent the rise.
- Check a clozapine level and consider reducing the dose when a patient quits.
- Plan for levels to fall if the patient resumes smoking after discharge.
Why it matters
A smoke-free admission can turn a stable clozapine dose into a toxic one.
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