Moderate to severe postoperative pain is relatively common after major abdominal surgery. It is associated with less than optimal surgical experience, poor quality of recovery, and the development of persistent postsurgical pain. Opioids remain a significant component of postoperative pain management. Side effects of opioids used for the treatment of postoperative pain include constipation, pruritus, nausea, and vomiting. Enhanced recovery after surgery (ERAS) protocols involve the utilization of multimodal analgesia. Analgesic techniques used include epidural analgesia, nerve blocks, and Intrathecal (IT) administration of morph ne. IT morphine reduces the postoperative opioid requirement for 18-24 hours after major abdominal surgery and reduces hospital length of stay (LOS) compared with epidural analgesia. A significant number of patients who receive IT morphine still experience moderate to severe postoperative p in. Additionally, many patients refuse the invasive procedure or cannot receive IT morphine due to procedure contraindications, thrombocytopenia, and/or coagulopathy. Intravenous (IV) methadone has a long analgesic half-life and has N-methyl-D-aspartate (NMDA) receptor antagonist and serotonin and norepinephrine reuptake inhibitor (SNRI) properties. It has previously been shown to reduce postoperative opioid requirements, postoperative nausea and vomiting (PONV), and postoperative pain scores in patients who underwent orthopedic, abdominal, complex spine, and cardiac surg ry. Similar findings have been shown in obstetric patients who underwent cesarean delivery under general anesthesia as well as patients who underwent gynecologic surgery. IV methadone has, however, never been compared with IT morphine as a postoperative analgesic. The hypothesis is that intravenous (IV) methadone is non-inferior to IT morphine in patients who undergo major abdominal surg ry. It offers the advantage of being a noninvasive analgesic modality that may contribute to decreasing opioid consumption during the first 72 hours postoperatively, controlling postoperative pain, and improving the quality of recovery after surgery.