Objective: The antidepressant escitalopram is predominantly metabolized by the polymorphic CYP2C19 enzyme. The authors investigated the effect of CYP2C19 genotype on exposure and therapeutic failure of escitalopram in a large patient population. Method: A total of 4,228 escitalopram serum concentration measurements from 2,087 CYP2C19-genotyped patients 10–30 hours after drug intake were collected retrospectively from the drug monitoring database at Diakonhjemmet Hospital in Oslo. The patients were divided into subgroups based on CYP2C19 genotype: those carrying inactive (CYP2C19Null) and gain-of-function (CYP2C19*17) variant alleles. The between-subgroup differences in escitalopram exposure (endpoint: dose-harmonized serum concentration) and therapeutic failure (endpoint: switching to another antidepressant within 1 year after the last escitalopram measurement) were evaluated by multivariate mixed model and chi-square analysis, respectively. Results: Compared with the CYP2C19*1/*1 group, escitalopram serum concentrations…