Interindividual variation of plasma haloperidol concentrations and the impact of concomitant medications: The analysis of therapeutic drug monitoringdata

We analyzed therapeutic drug monitoring (TDM) data from 231 schizophrenic i npatients (137 men, 94 women) and investigated interindividual differences of plasma haloperidol (HAL) concentrations and drug/drug interactions betwe en HAL and various concomitant drugs. Plasma HAL concentrations were determ ined by an enzyme immunoassay (EIA) method. Plasma HAL concentrations per d aily dose of HAL per body weight (HAL C/D ratio) demonstrated an approximat ely 11-fold interindividual variation. The patient subjects who received ca rbamazepine (CBZ) concomitantly had a mean HAL C/D ratio that was 37% lower than that of the patient subjects without CBZ. The patient subjects treate d with concomitant phenobarbital (PB) also showed a mean HAL C/D ratio that was 22% lower than those without PB. We concluded that careful evaluation of HAL TDM data and consideration of the impact of concomitant medication s uch as CBZ or PB that might influence the metabolism of HAL is necessary in daily clinical settings to avoid insufficient clinical response because of lowered concentrations of HAL or adverse effects because of high concentra tions of HAL.