Wilder-Smith et al.  recruited 50 women scheduled for abdominal hysterectomy for a randomized double-blind study to compare the effects of morphine and tramadol used for postoperative analgesia. Two weeks before surgery, all patients (ASA I–III) underwent tests to assess pain scores, rectal distension and skin electrical sensation, pain tolerance thresholds, and gastrointestinal transit times. The battery of tests was repeated on the second postoperative day and 1 month after surgery. Patients were premedicated with diazepam 10 mg orally 2 h before surgery and received a routine isoflurane- or halothane-based anesthetic, with a small dose of fentanyl administered at the beginning of the procedure. Surgical incisions were infiltrated with 20 ml 0.25% bupivacaine. Patients were then randomized to receive either tramadol or morphine in a continuous infusion postoperatively. The patients in the tramadol group received a loading bolus at time of wound closure of 2 mg/kg by slow intravenous injection, then an intravenous infusion pump of 0.5 mg · kg−1· h−1for the first 24 h and 0.25 mg · kg−1· h−1for the next 24 h. The loading bolus of morphine at the time of wound closure was 0.1 mg/kg intravenously, followed by continuous intravenous infusion of 0.05 mg · kg−1· h−1in the first 24 h and 0.025 mg · kg−1· h−1in the next 24 h.

Pain intensity scores decreased similarly in patients who received either morphine or tramadol, and pain tolerance thresholds in the incisional dermatome were similar with both drugs. At the 1-month follow-up, researchers observed no visceral sensitization in either group of patients. However, during morphine infusions, pain tolerance thresholds in the shoulder (included in the dermatome receiving visceral input via  the C5 spinal segment) increased in the first few postoperative days. The postoperative return of gastrointestinal and bowel function was more rapid with tramadol than morphine.