When smoking cessation occurs, what may happen to antipsychotic drug levels?

• A. Drug levels decrease
• B. No effect on levels
• C. Drug levels may rise and require dose adjustments
• D. Levels unpredictable and not adjust

Answer: (C)

Smoking induces the liver enzyme CYP1A2, which metabolizes several antipsychotics (notably clozapine and olanzapine). When a person is smoking, this induction increases the drug's clearance, keeping plasma levels lower. If smoking stops, the induction fades and metabolism slows, so the same dose now leads to higher plasma concentrations. This rise in drug levels can increase effects and risk of toxicity, so dose adjustments are often needed after cessation. Clinicians typically monitor drug levels and clinical response, and may reduce the dose if levels rise, with the return to baseline enzyme activity occurring over days to weeks.