Perioperative Rectal Methadone in Spine Surgery

Inclusion Criteria: 1. 18 to 65 years-old. 2. Elective spinal surgery with fusion of one or more sacral, lumbar, thoracic, and/or cervical levels 3. Capacity to provide informed consent 4. For participants of childbearing potential, use of contraception. Exclusion Criteria: 1. American Society of Anesthesiologists Physical Status \> IV 2. Hypersensitivity to the active substance (methadone hydrochloride) or other opioid analgesics or to any ingredient in the formulation 3. Pregnant or nursing participants 4. Known or suspected mechanical gastrointestinal obstruction 5. Acute respiratory depression, elevated carbon dioxide levels in the blood, cor pulmonale, or pulmonary disease necessitating home oxygen therapy 6. Acute alcohol intoxication, delirium tremens, and convulsive disorders secondary to alcohol intoxication 7. Severe central nervous system depression, increased intracranial pressure, or head injury 8. Use of monoamine oxidase (MAO) inhibitors, such as isocarboxazid, phenelzine, selegiline, and tranylcypromine, within 14 days of enrollment 9. Diarrhea associated with pseudomembranous colitis caused by cephalosporins, lincomycins, or penicillins 10. Preoperative renal insufficiency or failure 11. Significant liver disease (cirrhosis or hepatic failure) 12. History of opioid use disorder within the last 3 months 13. Patients taking more than 90 mg of morphine equivalents daily 14. Poor comprehension of the English language 15. Patients who are likely to remain intubated postoperatively 16. QT interval \> 500ms on preoperative ECG