Interaction between phenobarbital and thioridazine

The effects of thioridazine (TDZ) on serum phenobarbital (PB) and phenytoin (PHT) were studied in mentally retarded persons. Persons taking these drug combinations were identified and matched on the basis of age, sex, body weight, and antiepileptic drug (AED) dosage with persons from the same population who were taking only the AEDs. Trough, steady state serum levels were obtained as part of regular AED monitoring schedules. Serum level of PB per unit dose was significantly less with concomitant administration of TDZ (100 to 200 mg/day) than when PB was given alone. This effect seemed to be TDZ dose-responsive. TDZ had no consistent effects on serum PHT.