There is hope that biomarkers will be found to guide treatment selection. It might be of decisive interest to be able to assess an individual's metabolism activity. We propose here to explore the relationship between the activity of drug-metabolizing enzymes (DME) and transporters- assessed by a phenotypic approach and the efficacy of antidepressants. We will focus on venlafaxine (V) that provides a reasonable second-step choice for patients with depression and is used extensively in psychiatric practice, and the metabolism of which involves several cytochromes (CYP) P450 enzymes and the transporter P-gp.