RecruitingPhase 2Drug trialA Study to Evaluate TNX-102 SL Monotherapy Versus Placebo in Participants With Major Depressive Disorder (MDD)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