In this study, the blood levels of donepezil and risperidone, which are mainly metabolized by CYP2D6 and/or CYP3A, and the primary metabolites of both drugs were compared before and after an 8‐week treatment with yokukansan (7.5 g/day) in patients with dementia and schizophrenia. The gene discussed is CYP2D6; the disease is dementia.