Objective: Antidepressant discontinuation is a common problem in treating major depressive disorder (MDD). We examined the effects of antidepressant-related direct-to-consumer advertising (DTCA) and detail spending on medication switching, acute phase completion, and continuation phase completion among antidepressant users newly diagnosed with MDD. Methods: A retrospective cohort study of medical and prescription claims from a large national health plan affiliated with i3 Innovus from 2000 to 2004 was conducted. A total of 5010 individuals with MDD were identified. Antidepressant switch, acute phase completion, and continuation completion were determined by prescription refill records. Logistic regressions were run with DTCA and detailing spending variables as main explanatory variables. Results: In the switch model, own product detailing spending was negatively associated with the likelihood of switching (OR=0.61, 95% CI: 0.53-0.69). Own product detail spending was also positively associated with acute phase antidepressant completion (P<0.05). Other product DTCA was positively associated with continuation phase completion. Conclusion: Pharmaceutical spending on physician detail appears to impact antidepressant switching and completion of acute phase treatment by patients, while DTCA spending appears to have its greatest impact on successful completion of the continuation phase of antidepressant treatment.