Phase Ib/2a Drug-drug Interaction Study of a Combination of 45mg Dextromethorphan With 105 mg Bupropion

Inclusion Criteria: * Males and female subjects between 18 - 65 years of age; * Understand the study procedures and provide written informed consent in the English language. * Meet current DSM-5 criteria for OUD, of at least moderate severity, currently engaged in MOUD treatment at a buprenorphine-naloxone sublingual film total daily dose ranging from 8mg/2mg to 24mg/6mg or buprenorphine sublingual tablet 5.7mg/1.4mg to 17.1/4.3 daily for at least 2 weeks at screening. Or on a stable dose of depot injectable buprenorphine for at least four months, with at least one week since last depot buprenorphine injection. * Have a positive urine drug screen for buprenorphine during screening and upon presenting for the first laboratory day on the clinical research unit to document buprenorphine use; * Quick Inventory of Depressive Symptomatology (16-Item) (QIDS-SR16) score of mild or greater (\>6) * Females must be non-pregnant and non-lactating. Additionally, for females with childbearing potential (ie., have not undergone sterilization via hysterectomy, bilateral tubal ligation, or bilateral oophorectomy, or at least 1 year post-menopausal), participants must agree to use an acceptable form of contraception during study participation and to continue its use for at least 30 days after the last dose of the study drug (e.g, abstinence, intrauterine device, hormonal implant, hormonal patch/ring/pill, condoms (male or female). Exclusion Criteria: * Contraindications for participation as…