A pharmacokinetic and pharmacodynamic evaluation of the combined administration of alprazolam and fluvoxamine.

We have assessed the pharmacokinetic and pharmacodynamic interaction between fluvoxamine, a serotonin reuptake inhibitor, and alprazolam, a triazolobenzo-diazepine. Healthy men took fluvoxamine maleate daily for 10 days (50 mg on days 1-3, 100 mg on days 4-10) (n = 20), 1 mg of alprazolam four times daily for four days (days 7-10 of the study period) (n = 20), or a combination of the two (n = 20), according to a parallel study design. Alprazolam and fluvoxamine concentrations were measured in serial plasma samples by HPLC and gas chromatography respectively, and psychomotor performance and memory were assessed on days 1, 7, and 10. Fluvoxamine increased plasma alprazolam concentrations by 100%. The mean apparent half-life of alprazolam was increased from 20 h to 34 h after fluvoxamine co-administration. The increased plasma concentrations of alprazolam resulted in significantly greater reductions in psychomotor performance evident on day 10. Mean fluvoxamine plasma concentrations were about 25% lower in those who took the combination than in those who took only fluvoxamine; this was more likely due to heterogeneity between the treatment groups than to an effect of alprazolam. The dosage of alprazolam should be reduced during co-administration with fluvoxamine.