ObjectiveTo evaluate the relationship between adherence to antidepressant therapy and economic outcomes.DesignRetrospective database analysis using pharmacy and medical claims from a pharmacy benefit and medical management company serving a large managed care organization (MCO) that provides healthcare coverage for approximately 3.5 million members.ParticipantsPatient selection was based on the following criteria:newly started on antidepressant therapy between 1 April 1999 through 30 June 1999;titrated to a usual antidepressant dosage level within 6 months of the initiation of therapy;continuously enrolled in the health plan between 1 January 1999 through 31 December 1999; and>18 years of age.Outcome measuresAntidepressant adherence was calculated as a ratio of the total number of day’s supply during the 180-day follow-up period divided by 180 days. Patients were defined as adhering to treatment if they had a ratio of ≥0.70. The means of pharmacy ingredient costs, medical charges and total healthcare charges incurred during the follow-up period were compared. Adjusted means (least squares means) were calculated after adjusting for potential confounding factors that may have influenced relevant outcomes.ResultsOf the total cohort (14 190 patients), 39.7% (n = 5638) of patients were deemed to be adhering (≥70.0% completion) to their treatment. Adherent patients were significantly more advanced in age (55.2 vs 54.3 years, p < 0.01) and had a higher mean Chronic Disease Score (3.80 vs 3.47, p < 0.0001). After adjusting for confounding factors, adherent patients incurred lower total healthcare charges ($US11 327 vs $US11 815, p = 0.433) significantly lower medical charges ($US9411 vs $US10 692, p = 0.039) and significantly higher pharmacy charges ($US1915 vs $US1123, p < 0.0001) than non-adherent patients during the initial 6 months of therapy (all 1999 values).ConclusionIn this MCO, patients who were adherent with antidepressant therapy possessed significantly lower medical charges. These findings indicate that patient adherence with antidepressant therapy significantly improved the economic outcomes. It is suggested that there is a need for raising awareness about the importance of patient adherence as well as to improve methods of detecting individuals with depression in order to gain the economic benefits associated with adherence.