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

Although reported for Caucasians, changes in plasma dozapine levels after smoking cessation in East Asians remain unclear. We here investigated plasma dozapine 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 dozapine 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 dozapine levels between before and after the smoking ban, which were compared using paired t-tests. The mean plasma dozapine level was significantly increased, by 213.4 ng/mL (95% CI 119.9-306.8; p < 0.01) or 53.2%. Four of the 14 inpatients experienced clinically significant side effects, such as myodonus, drooling, and amnesia, due to the development of high plasma dozapine levels. Our findings indicated that close monitoring of plasma dozapine levels before and after smoking cessation and prior dose adjustment of dozapine may be necessary, to prevent a significant risk of developing high plasma dozapine levels, even in Japanese patients.