Clozapine serum concentrations are lower in smoking than in non-smoking schizophrenic patients.

Serum concentrations of clozapine and its main metabolite demethylclozapine were measured in 44 schizophrenic inpatients, of whom ten were non-smokers and 34 smokers. When comparing their clozapine dose and body weight-related serum drug levels, we found that clozapine and demethylclozapine concentrations were about 40% lower in the smoking than in the non-smoking group, probably due to an inducing effect of smoking on the cytochrome P450 (CYP) 1A2, which is involved in the metabolism of clozapine. We conclude that dosage adjustment may be necessary in clozapine-treated smokers.