Venlafaxine is a model substrate for the drug metabolizing cytochrome P450 (CYP) enzyme 2D6. The desvenlafaxine/venlafaxine ratio, either after a single dose or at steady state, can be used to determine whether a patient is functionally (i.e., phenotypically) a CYP 2D6 extensive or poor metabolizer (EM or PM). In turn, CYP 2D6 EM and PM status is important in determining the efficacy of venlafaxine as an antidepressant. Based on a secondary analysis of four of the venlafaxine registration trials, venlafaxine was effective in patients who were CYP 2D6 EMs versus a parallel placebo-treated control group, whereas it was not effective in patients who were CYP 2D6 PMs. Thus, venlafaxine is a useful example of how drugs can be used to quantify differences in drug metabolizing capacity among patients and how such differences can in turn affect the efficacy of a drug (i.e., make a patient an outlier on the usual dose-response curve).