We investigated the effects of age, sex, and genetic polymorphisms in CYP isoforms (CYP1A2, CYP2B6, CYP2C19, CYP2D6, and CYP3A5) and drug transporters (ABCG2 and SLCO1B1) on the plasma concentrations of clozapine, N‐desmethylclozapine, and clozapine N‐oxide in Japanese patients with schizophrenia. This evidence concerns the gene CYP2D6 and schizophrenia.