Major depressive disorder (MDD) is the leading cause of disability worldwide and reduces life expectancy. Achieving and sustaining remission from depression is challenging after initial improvement of an acute episode with an antidepressant, especially for patients whose depressive episodes have proven treatment-resistant in response to conventional antidepressant pharmacotherapy. While standard antidepressants are at least partly effective for the short-term treatment of acute depressive episodes of MDD, many patients relapse within 6 months of apparent clinical remission, with faster and higher rates observed in those with treatment-resistant depression (TRD). Efficacy of IV ketamine, a rapid-acting N-methyl d-aspartate (NMDA) receptor antagonist, in maintaining antidepressant effect was suggested in a few small, single center, open-label studies and case series. More recently, maintenance of antidepressant effects beyond the initial acute (induction) treatment period has been shown with esketamine nasal spray, an enantiomer of ketamine, in conjunction with an oral antidepressant in three phase 2/3 registration studies (SYNAPSE, SUSTAIN-1, SUSTAIN-2) of adult patients with TRD. In these studies the maintenance of efficacy of an intermittently-dosed esketamine treatment regimen was established in which twice-weekly dose administration during a 4-week induction period was followed initially by weekly administration and later by either weekly or every-other-week administration. During long-term maintenance therapy the antidepressant effect persisted in most patients with this regimen, despite their history of being resistant to conventional antidepressants prior to entering esketamine studies. These data suggest that the neurobiological changes induced by initial esketamine treatment, which putatively underlie its antidepressant effect, can be maintained using repeated administration.