Chlorzoxazone is a centrally acting muscle relaxant used to treat muscle spasm and the resulting pain or discomfort. It acts on the spinal cord by depressing reflexes. Our purpose is to investigate the effect of chlorzoxazone on moderate to severe postoperative pain after spine surgery. Our hypothesis is that chlorzoxazone can reduce postoperative pain and reduce opioidconsumption and side effects compared to placebo.

Inclusion Criteria:

• Patients undergoing spine surgery in general anaesthesia.
• Postoperative pain > 50 mm on the VAS scale during mobilization.
• Patients who have not received analgesia 1 hour prior to inclusion.
• ASA 1-3.
• BMI > 18 og < 40.
• Fertile women need a negative HCG urine test.
• Patients who have given their written consent to participate and understand the contents of the protocol.

Exclusion Criteria:

• Participation in another clinical trial.
• Patients who do not speak and/or understand Danish.
• Fertile women with a positive HCG urine test.
• Allergy to the drugs used in the trial.
• Alcohol or medicine abuse, assessed by investigator.
• Daily use of strong opioids (morphine, ketobemidone, oxynorm, methadone, fentanyl)
• Daily chlorzoxazone treatment.
• Known or suspected porphyria.