The aim of this study was to investigate the dosing patterns of adjunctive quetiapine or adjunctive aripiprazole in the treatment of major depressive disorder from 2006 to 2010, and to evaluate the impact of Food and Drug Administration (FDA) approval on these dosing patterns. Patients included in the study were adults diagnosed with major depressive disorder, and treated with adjunctive aripiprazole or quetiapine between the years 2006 and 2010. The average daily dose and dose distribution were calculated and assessed statistically over the same time period. The mean daily dose for patients treated with adjunctive aripiprazole decreased from 13.5 mg/day in 2006 to 6.9 mg/day in 2010, whereas the mean daily dose for patients treated with quetiapine increased from 129 mg/day in 2006 to 139 mg/day in 2007, decreasing to 123 mg/day in 2010. The proportion of patients receiving FDA-recommended doses increased significantly for aripiprazole (86.3% in 2006 to 94.5% in 2010; P<0.001) and remained relatively stable for quetiapine (21.3% in 2006 to 24.0% in 2010; NS). The majority of patients treated with quetiapine received doses below those recommended by the FDA throughout the study period. Aripiprazole was mostly prescribed at therapeutic doses (pre-FDA and post-FDA approval), although the mean dose decreased significantly over time.