• Male, female or non-binary, age 18 to 65 years of age at Screening.

• Diagnosis of Opioid Use Disorder (OUD)

• Positive Urine Drug Screen (UDS) for opioid(s) at the Screening and Baseline Visits.

• Recent active/chronic use of short-acting illicit and/or prescribed opioids and/or long-acting Opioid Use Disorder (OUD) maintenance treatments buprenorphine or methadone at the Screening and Baseline Visits.

• Already engaged and fully assessed in a longitudinal-outpatient treatment program that provides opioid addiction treatment encompassing the full spectrum of opioid maintenance and abstinence (injectable Vivitrol®) treatments, in which the host clinic is prepared and equipped to continue with:

∙ maintenance treatment (methadone or buprenorphine treatment) for study non-completers, or

‣ long-acting injectable naltrexone treatment (Vivitrol®), for completers with next dose delivered approximately 30 days after Study Day 6.

• Post-menopausal/sterile or agree to use chemical or barrier methods of birth control from time of informed consent through 30 days post last treatment.

• Stable concomitant medications.

• Stable concomitant medications for depression, post-traumatic stress disorder, psychotic disorders, and bipolar spectrum disorders if one of the following: SSRI, SNRI, bupropion, MAOI, trazodone, Tricyclic, typical and atypical antipsychotics, lithium, antihistamine, alpha-adrenergic agent, nicotine replacement.

• Stable concomitant medications: propranolol, prazosin, and clonidine if used for psychiatric reasons, and not to control hypertension at the Baseline Visit and unchanged on Study Day 1.

⁃ Provide informed consent.

⁃ Understand and follow Lifestyle Considerations per protocol.