A Trial of the Efficacy and Safety of Fixed Doses of SEP-363856 in Adults With Generalized Anxiety Disorder

Key Inclusion Criteria: 1. Male or female participants at least 18 years of age at the time of informed consent 2. Primary diagnosis of Generalized Anxiety Disorder (GAD) per Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), confirmed by Mini-International Neuropsychiatric Interview (MINI) and clinical psychiatric evaluation 3. Body Mass Index (BMI) between 18 and 40 kilograms per square meter (kg/m\^2) (inclusive) 4. Participant is willing and able to comply with study procedures and visit schedule 5. Medically stable as determined by medical history, physical exam, vital signs, and laboratory tests 6. Willing and able to discontinue prohibited medications/therapies per protocol 7. i) Females of childbearing potential: negative pregnancy test and agreement to use acceptable contraception ii) Males: agreement to follow protocol-defined contraception and reproductive restrictions 8. Participant is willing and able to safely discontinue all prohibited medications/therapies/herbal supplements in accordance with the protocol-required washouts prior to baseline/Day 1 and during the trial period. Key Exclusion Criteria: 1. Current DSM-5 diagnosis of major depressive episode, post traumatic stress disorder (PTSD), or any psychiatric disorder other than GAD that was the primary focus of treatment within 12 months prior to screening 2. Substance use disorder (excluding nicotine/caffeine) within 12 months prior to screening 3. Lifetime history of certain psychiatric disorders (e.g., schizophrenia spectrum disorders, bipolar disorder, obsessive-compulsive disorder, borderline or antisocial personality disorder) 4. Significant suicide risk (e.g., recent suicidal behavior/ideation per Columbia-Suicide Severity Rating Scale \[C-SSRS\] or MADRS) 5. Use of prohibited medications/therapies within protocol-defined timeframes (e.g., certain antipsychotics, mood stabilizers, benzodiazepines, investigational treatments) 6. Prior exposure to restricted treatments 7. Pregnant or breastfeeding females 8. Participation in another investigational study within 180 days (or more than \[\>\] 1 trial in last year) 9. Known history of human immunodeficiency virus seropositivity, hepatitis B or C