Because concomitant administration of psychoactive and antiepileptic drugs is increasing progressively in neurologic and psychiatric practice, the aim of the current study was to evaluate the pharmacokinetic interactions between risperidone (RISP) and carbamazepine (CBZ) plasma concentrations in a group of patients with epilepsy with behavioral disturbances. The authors assessed eight patients on CBZ monotherapy (CBZ extended-release capsules) at a mean dosage of 625 +/- 253 mg/day (range, 400-1,200 mg/day) for at least 1 year. RISP (1 mg in one daily dose) was added to CBZ therapy for the occurrence of behavior disturbances. CBZ blood levels were assessed before (T0), 24 hours after (T1), and 2 weeks after (T2) RISP administration. Steady-state plasma concentrations of CBZ increased from 6.67 +/- 0.41 microg/mL at baseline to 7.37 +/- 0.59 microg/mL (p < 0.01) at T1, to 7.95 +/- 0.47 microg/mL (p < 0.0001) at T2. The pharmacokinetic data suggest either a possible role of RISP in inhibiting the cytochrome P450 microsomal enzyme system (CYP)-3A4 pathway or a potential role of CYP2D6 in CBZ metabolism.