The goal of this clinical trial is to learn if a drug called TNX-102 SL works to treat moderate to severe major depressive disorder in adults. It will also learn about the safety of TNX-102 SL. The main questions it aims to answer are:

Does TNX-102 SL improve depression symptoms according to a depression symptom rating scale? What medical problems do participants have when taking TNX-102 SL?

Researchers will compare TNX-102 SL to a placebo (a look-alike substance that contains no drug) to see if TNX-102 SL works to treat major depressive disorder.

Participants will:

Take TNX-102 SL or a placebo every night at bedtime for 6 weeks Visit the clinic once every 2 weeks for checkups and tests

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)