This randomized, double-blind controlled trial investigates whether intraoperative intravenous methadone (0.15 mg/kg based on ideal body weight) reduces acute postoperative pain and opioid consumption in patients undergoing elective total hip arthroplasty under spinal anesthesia with mepivacaine. The primary outcome is patient-reported pain on a visual analog scale (VAS) 30-60 minutes after arrival in the post-anesthesia care unit (PACU). Secondary outcomes include opioid use, pain scores over time, incidence of nausea/vomiting, and quality of recovery. A total of 162 subjects will be enrolled at MUSC surgical sites.