Interindividual variation of serum haloperidol concentrations in Japanese patients – clinical considerations on steady-state serum level–dose ratios.

Summary Objective: Marked interpatient variability in haloperidol (HAL) level–dose (L/D) ratios makes it difficult to use the administered dose for predicting serum concentrations. Objective: To investigate the effect of dose, age, total body weight and co-medication on steady-state HAL L/D ratios. Method: Retrospective analysis of dose and HAL blood level data from 168 patients. Results: The HAL L/D ratio decreased curvilinearly with increasing daily dose of HAL. The patients treated with concomitant antiparkinsonian drugs showed a mean HAL L/D ratio that was 24·9% higher than those without antiparkinsonian drugs. The patients treated with concomitant antiepileptic drugs showed a mean HAL L/D ratio that was 27·2% lower than those without antiepileptic drugs. The mean HAL L/D ratio of patients treated with concomitant CYP2D6 substrates was not significantly different from those without CYP2D6 substrates. Conclusion: There is a wide interindividual variability in blood levels of HAL in patients given the same dose. Routine monitoring of HAL serum level is useful, especially in patients who require associated antiepileptic and/or antiparkinsonian medication. [ABSTRACT FROM AUTHOR]