Changes in Plasma Clozapine Levels after Smoking Cessation in Japanese Inpatients with Schizophrenia: A Retrospective Cohort Study

Although reported for Caucasians, changes in plasma clozapine levels after smoking cessation in East Asians remain unclear. We here investigated plasma clozapine levels before and after smoking cessation in Japanese inpatients with schizophrenia. We conducted a retrospective chart review of 14 inpatients with schizophrenia who were being treated with clozapine between June 1, 2019, and July 31, 2019 and who were smokers as of July 1, 2019, the day on which a smoking ban was instituted in the tertiary public psychiatric hospital. The primary outcome was individual differences in plasma clozapine levels between before and after the smoking ban, which were compared using paired t-tests. The mean plasma clozapine level was significantly increased, by 213.4 ng/mL (95% CI 119.9-306.8; p