Patients undergoing spinal surgery require pain control medication after their surgery. Investigators have successfully used intravenous Methadone to manage pain after surgery. However, doctors in Canada do not have the intravenous form of Methadone to prescribe to their patients. The investigators in Canada propose a pilot trial to investigate whether Methadone administered rectally could be used to manage pain after spinal surgery.

The main questions are:

1. Are investigators able to recruit participants for this trial and learn from this study to plan a larger trial?
2. Does Methadone administered rectally during surgery, reduce participants' pain intensity, use less pain medication, and have a better recovery after surgery? Investigators will compare Methadone to a placebo (a look-alike substance that contains no drug) to see if Methadone works to manage pain after surgery better than the usual pain management.

Participants will:

• receive either Methadone or placebo during surgery.
• be asked some questions about their pain during days 1 to 3 after surgery
• be contacted by phone to ask about their recovery

At this time, the study aims to recruit 40 participants from St. Michael's Hospital, to learn whether it will be feasible to plan a larger study.

Indication: Adult patients undergoing spine surgery Condition: Perioperative pain management Number of participants 40 Primary outcome Feasibility (recruitment success, consent rate, adherence, patient withdrawal, missing data, adverse outcomes) Secondary outcome 1) Average pain intensity; 2) Rate of respiratory depression and postoperative ileus; 3) Use of opioids in morphine equivalents; 4) Quality of recovery Study design

• Interventional trial
• Allocation: Randomized
• Intervention model: 2-Arm Parallel-Group
• Primary purpose: Feasibility
• Phase: Phase IV

Masking Participants and Outcome Assessors

Study Intervention:

• Intervention Arm: Methadone 0.2mg/kg of ideal body weight via rectal administration + standard or care
• Placebo Arm: Saline solution via rectal administration + standard of care

Follow-Up: Postoperative days 1, 2, 3, and 30

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

This is a feasibility pilot trial only.