OBJECTIVES: For patients with major depressive disorder (MDD), adjunctive therapy is often a second treatment step following a partial response to an antidepressant.  Although the role of adjunctive treatment is supported in practice guidelines, there is little information regarding the actual practice of adjunctive therapy, particularly for patients seen in primary care. The objectives of the study were to examine the incidence and predictors of adjunctive pharmacotherapy among patients with MDD treated with selective serotonin reuptake inhibitors (SSRIs) by primary care physicians (PCPs) in the United Kingdom (UK).  METHODS: The General Practice Research Database was used to identify 15,274 patients with MDD who were prescribed first-line treatment with SSRIs from 2006-2008.  Treatment trajectories were identified and classified as adjunctive therapy, drug switches, dose increases, discontinuation, and restart of therapy. Incidence and predictors of adjunctive therapy were assessed. Comparisons in healthcare resource utilization were made between patients receiving adjunctive therapy and patients receiving other treatment strategies. RESULTS: Overall incidence of adjunctive therapy was 3.07/100 person years (95% CI 2.90-3.25). Patients prescribed adjunctive therapy were more likely to be female (IRR 1.17, p=0.02), of higher age (IRRs 1.53-2.41, p<0.001), and had greater depression severity (IRR 1.02, p=0.004). Presence of postherpetic neuropathy (IRR 3.06, p=0.01), irritable bowel syndrome (IRR 1.33, p=0.01), and an increasing Charlson Comorbidity Index (IRR 1.08, p=0.03) were associated with a higher incidence of adjunctive therapy. MDD-related general practitioner consultations were lower among those prescribed adjunctive therapy compared with patients receiving other treatment interventions (IRRs 0.79-0.87, p<0.001). CONCLUSIONS: Incidence of adjunctive treatment was relatively low and was associated with several patient demographics, a higher burden of illness, and less PCP visits.  The incidence of adjunctive therapy suggests that it is infrequently used in the management of MDD among patients who are partial responders to SSRI treatment in UK primary care.