Inclusion Criteria:  Have a diagnosis of moderate to severe major depressive disorder (MDD) and/or post-traumatic stress disorder (PTSD) At baseline, either not taking an antidepressant medication, or currently taking a SSRI, SNRI, mirtazapine, or bupropion for at least 6 weeks with no dose modifications in the past 2 weeks Willing to comply with all study assessments and procedures Must not be pregnant or breastfeeding at time of enrollment or throughout study  Exclusion Criteria:  Evidence of unstable cardiovascular, respiratory, liver, or renal impairment or disease Active suicidal ideation Diagnosed bipolar disorder, psychotic disorder, or dementia Current moderate or severe substance use disorder Has a history of hypersensitivity or allergic reaction to ALTO-100 or any of its components/excipients Concurrent or recent participation in another clinical trial for mental illness involving an investigational product or device