Carbamazepine significantly reduced the plasma concentration of paliperidone. The plasma concentration of paliperidone at baseline and with coadministration of 200, 400, and 600 mg/d were 45.8 ± 11.7, 26.9 ± 13.7, 17.1 ± 8.2, and 15.9 ± 7.6 ng/mL, respectively. The concentration of paliperidone with carbamazepine coadministration at doses of 200, 400, and 600 mg/d were 55.7% ± 20.7%, 36.1% ± 12.2%, and 33.6% ± 10.4%, respectively, of baseline. This effect occurred even at the carbamazepine dose of 200 mg/d and reached a plateau at doses higher than 400 mg/d. However, carbamazepine coadministration exacerbated the psychotic symptoms in some patients.