Three Dosing Regimens of Dextromethorphan (DXM) Reportedly Used in Major Depressive Disorder

Inclusion Criteria: * Subject must be a man or woman, 18 to 50 years, inclusive * Subject must meet Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-V) criteria for MDD without psychotic features based on clinician assessment. * Subjects must have undergone 3 or fewer antidepressant trials * Subjects must have moderate to severe depression as determined by a screening Montgomery-Åsberg Depression Rating Scale (MADRS) score of 20 or greater Exclusion Criteria: * Subject has a current diagnosis of Bipolar Affective Spectrum Disorder, obsessive compulsive disorder, or a DSM-V personality disorder * Subject has had an incomplete response to more than 3 antidepressant trials * Subject cannot tolerate 60mg DXM as determined by study physician * Subject meets DSM-5 severity criteria for moderate or severe substance or alcohol use disorder within 6 months of screening, excluding caffeine or nicotine * Subject has a current clinical diagnosis of autism, dementia, or intellectual disability * Subject has history of psychosis not attributable to organic/metabolic causes * Subject has a history of serotonin syndrome or sensitivity * Subject has taken venlafaxine or a monoamine oxidase inhibitor (MAOI) within 4 weeks prior to screening * Subject is actively taking an opiate antagonist for any indication * Subject is taking a higher-than-recommended dosage of serotonergic antidepressant \[including but not limited to selective serotonin reuptake inhibitor (SSRI), serotonin-norepinephrine reuptake inhibitor (SNRI), serotonin partial agonist/reuptake inhibitor (SPARI), serotonin antagonist and reuptake inhibitor (SARI) within 4 weeks prior to screening * Subject is actively suicidal * Subject has participated in another clinical drug trial within 6 months prior to screening * Subject has a history or current signs of liver or renal insufficiency * Subject has a clinically significant condition of any organ system that might adversely impact subject health upon consumption of DXM * Subject has previous history of intolerance to ketamine or its enantiomers * Subject has previous history of incomplete antidepressant response to ketamine or its enantiomers * Subject is currently using DXM for any indication