Facilitating Rapid Naltrexone Initiation

Eligibility Details: Inclusion Criteria: 1. DSM-5 criteria of current opioid use disorder present for at least six months, supported by a positive urine for opioids or a positive naloxone challenge test (MINI, psychiatric exam) 2. Aged 18 to 60 years (history) 3. In otherwise good health based on complete medical history, physical examination, vital signs measurement, ECG, and laboratory tests (hematology, blood chemistry, urinalysis) within normal ranges 4. Able to give written informed consent to participate in the study (psychiatric evaluation) 5. Interested in maintenance treatment with extended-release naltrexone Exclusion Criteria: 1. Physiologically dependent on alcohol or sedative-hypnotics with impending withdrawal requiring medical management. (psych eval, MINI) 2. Methadone maintenance treatment or regular use of illicit methadone (>30 mg per week); urine toxicology positive for methadone at admission (psych eval, urine tox) 3. Buprenorphine maintenance treatment or regular use of buprenorphine (>16 mg per week); urine toxicology positive for buprenorphine at admission (psych eval, urine tox) 4. Active, or past, psychiatric disorder which might interfere with participation or make participation hazardous, including DSM-V organic mental disorder, psychotic disorder, or bipolar disorder with mania (psych eval, MINI) 5. Significant current suicidal risk or a suicide attempt within the past year (psych eval) 6. On psychotropic or other medications that may interact adversely with study medications, or whose effect might be disrupted by study medications (psych/med eval, MINI) 7. Positive serum pregnancy test, lactation, or unwillingness to use a satisfactory method of birth control (med eval, bloodwork) 8. Uncontrolled neurological, cardiovascular, renal, and hepatic diseases, active tuberculosis, AIDS, or any other disorder that might make administration of study medications hazardous (med eval) 9. Acute hepatitis with SGOT or SGPT > 3 times the upper end of the laboratory normal range (bloodwork) 10. Concurrent participation in another treatment study or another substance abuse program with the exception of a self-help group (psych eval) 11. History of allergy or sensitivity to any study medication (med eval) 12. Ongoing chronic pain that may require opioid management, or for which surgery is indicated (med eval) 13. History of inability to tolerate study medications (psych eval) 14. History of ketamine use disorder (psych eval, MINI) 15. Active major depressive disorder (psych eval, MINI)