A Study to Evaluate TNX-102 SL Monotherapy Versus Placebo in Participants With Major Depressive Disorder (MDD)

Inclusion Criteria: * Primary DSM-5 diagnosis of current MDD. * The duration of the current MDE must be between 6 weeks and 18 months. * Without psychotic or catatonic features. * Capable of reading and understanding English and able to provide written informed consent to participate. Exclusion Criteria: * Diagnosis of DSM-5-defined lifetime bipolar disorder (I, II, or unspecified), schizophrenia, schizoaffective disorder, MDD with psychotic features, other psychotic disorder, or antisocial personality disorder; current (past month) obsessive-compulsive disorder; current (past month) posttraumatic stress disorder; current (past 3 months) anorexia nervosa, * Diagnosis of borderline personality disorder that is known, suspected * Participants with comorbid generalized anxiety disorder (GAD), social anxiety disorder (SAD), or panic disorder are excluded only if the GAD, SAD, or panic disorder is considered the primary psychiatric diagnosis, rather than MDD. * Participants with treatment refractory MDD, ie, previously having failed in their lifetime ≥2 treatments (due to inadequate efficacy) with at least 2 different classes of antidepressants of adequate dose, duration, and treatment adherence. * History of substance use disorder and/or alcohol use disorder during the preceding 12 months * Use of antidepressants (including ketamine/esketamine, St. John's Wort, S-adenosyl methionine, and/or trazodone used as an antidepressant) within 4 weeks of Baseline (Visit 2), except for fluoxetine, which must not be within 6 weeks of Baseline (Visit 2)