Dose-dependent interaction of paroxetine with risperidone in schizophrenic patients.

Augmentation with paroxetine (10-40 mg/d) for antipsychotic treatment may improve the negative symptoms in schizophrenic patients but involves a risk of drug-drug interaction. We studied the effects of paroxetine on plasma concentrations of risperidone and 9-hydroxyrisperidone and their clinical symptoms in risperidone-treated patients. Twelve schizophrenic inpatients with prevailingly negative symptoms receiving risperidone 4 mg/d were, in addition, treated with incremental doses of paroxetine for 12 weeks (10, 20, and 40 mg/d for 4 weeks each). Plasma concentrations of risperidone and 9-hydroxyrisperidone were quantified with liquid chromatography-mass spectrometry mass-mass spectrometry together with clinical assessments before and after each phase of the 3 paroxetine doses. Risperidone concentrations during coadministration of paroxetine 10, 20, and 40 mg/d were 3.8-fold (95% confidence interval, 3.2-5.8, P < 0.01), 7.1-fold (95% confidence interval, 5.3-16.5, P < 0.01), and 9.7-fold (95% confidence interval, 7.8-22.5, P < 0.01) higher than that before paroxetine coadministration, respectively. Active moiety (risperidone plus 9-hydroxyrisperidone) concentration was not increased during the paroxetine 10 mg/d (1.3-fold, not significant) or 20 mg/d (1.6-fold, not significant), but were significantly increased by 1.8-fold (95% confidence interval, 1.4-2.7, P < 0.05) during the paroxetine 40 mg/d. Significant improvement in negative symptoms was observed from 10 to 40 mg/d of paroxetine, whereas scores in extrapyramidal side effects during 20 and 40 mg/d of paroxetine were significantly higher than baseline score. This study indicates that paroxetine increases plasma risperidone concentration and active moiety concentration in a dose-dependent manner. Low-dose coadministration of paroxetine with risperidone may be safe and effective in the treatment of schizophrenic patients with negative symptoms.