All study sessions were conducted by a blinded research assistant (N.D.) and supervised by an unblinded physician (L.C.) who administered the study medication and monitored heart rate, blood pressure, and arterial oxygenation throughout the study. The Subjective Opioid Withdrawal scale (SOWS) and Objective Opioid Withdrawal scale (OOWS) were completed after establishing intravenous (i.v.) access and before administration of study medications, as originally described by Handelsman et al. [5]. All OOWS measurements were obtained by the same blinded research assistant (N.D.). Ondansetron (Bedford Laboratories, Bedford, Ohio, USA) 8 mg or placebo (0.9% saline solution, Hospira Inc., Lake Forest, Illinois, USA) was administered in a double-blinded manner as an i.v. bolus. Thirty minutes later, morphine (Baxter Healthcare Corporation, Deerfield, Illinois, USA) 10 mg/70 kg was administered over 10 min. Patients remained in the lab under observation for 105 min and were offered music or video entertainment and caffeine-free meals or snacks ad lib during this period of time. After 105 min, the patient's vital signs and OOWS and SOWS were reassessed. One hundred and twenty minutes after i.v. morphine administration, 10 mg/70
