In this study, we investigated the effects of variations in the CYP2C19, CYP2D6, and ABCB1 genes on treatment-related changes in SF and SS in patients with MDD on ESC monotherapy or ESC with ARI augmentation from the nationwide Canadian Biomarker Integration Network in Depression 1 (CAN-BIND-1) study.32 We hypothesized that IM or PM CYP2C19 and CYP2D6 metabolizer phenotypes, and ABCB1 genotypes associated with lower P-gp function or expression,33 would show a greater decline in SF and SS from baseline relative to those with normal activity. Here, CYP2C19 is linked to major depressive disorder.