OBJECTIVES: Major depressive disorder is characterised by the presence of one or more major depressive episodes. Up to one-third of patients do not adequately respond to first-line therapies. In case of treatment failure, the most common strategy is to switch to another antidepressant drug (AD). However, patients may have been exposed in prior episodes to one or more other ADs, which could impact the efficacy of subsequent treatments. This study describes historical AD use in depressed patients undergoing treatment switch. METHODS: This retrospective longitudinal cohort study used a database of medical records from general practitioners located throughout the UK (CPRD). Adult patients with a depression diagnosis undergoing an AD switch between 01/01/2012 and 30/06/2013 and with no diagnosis of schizophrenia or bipolar disorder were included. Historical AD use was assessed on all available prescriptions prior to switch and was grouped by treatment class (SSRI, SNRI, TCA and others). RESULTS: CONCLUSIONS: This analysis showed that for patients initiating an AD switch, historical use of ADs was very common, and that a substantial proportion of patients had already been prescribed several ADs, thereby reducing the treatment options for future switches.