Fluctuating plasma levels of the active moiety of risperidone is related to occurrence of extrapyramidal symptoms.

In the present study, we compared the plasma levels of the active moiety (i.e. risperidone plus 9-hydroxyrisperidone) in a steady state in 54 Japanese schizophrenic patients with or without emerging extrapyramidal symptoms (EPS and non-EPS groups, respectively) who were treated with 4 mg/day risperidone. No differences were observed in the plasma levels of the active moiety at 4 and 8 weeks after risperiodne administration in both groups. However, the EPS group patients showed a greater number of fluctuations/larger fluctuations in the plasma active moiety levels than those in the non-EPS group. These results suggest that a stable plasma active moiety level maybe important for preventing EPSs during treatment with risperidone.