Inclusion Criteria:  American Society of Anesthesiologists (ASA) physical status 1-3 patients undergoing spinal fusion surgery  Exclusion Criteria:  patients who refuse to participate in this study patients unable to communicate due to mental impairment or developmental delay patients allergic to acetaminophen or its additives or who are contraindicated in acetaminophen administration for other reasons patients with existing liver diseases or dysfunction (i.e. active hepatitis, clinically relevant chronic liver conditions, elevated liver enzymes) patients who are judged ineligible by the medical staff to participate in the study for other reasons