Approximately one-third of patients with major depressive disorder do not achieve adequate symptom control despite a series of multiple treatment trials with currently available antidepressant medication (for example a serotonin-selective reuptake inhibitor). This group of patients - representing treatment-resistant depression (TRD) - accounts for an alarmingly high public health burden and signifies a critical area of need in pharmaceutical treatment development. While current treatments are slow to act and only partially effective, new basic and clinical research focusing on the glutamate system is yielding promising new avenues for novel drug discovery. Ketamine - a glutamate N-methyl-d-aspartate (NMDA) receptor antagonist - has now been demonstrated in several studies to bring about a rapid and robust antidepressant effect, even in patients suffering from TRD. Ketamine is limited as a treatment for TRD by the need for intravenous administration and the potential for untoward medical or psychiatric adverse effects. There is an urgent need, therefore, to identify well-tolerated, orally available compounds that target the NMDA receptor as a novel treatment approach for TRD. The current project aims to test the safety, tolerability and efficacy of Nuedexta - containing the NMDA antagonist dextromethorphan.