Major depressive disorder (MDD) is common, recurrent, chronic, and disabling. Due in part to its prevalence, depression is globally responsible for more years lost to disability than any other disease. Up to 50% of individuals treated with antidepressant medications for MDD do not achieve full remission. Thus, the currently available treatments for depression do not effectively or sufficiently reduce the associated morbidity or mortality. To reduce the burden of MDD, more effective and adjunctive therapies are needed.