72 patients scheduled to undergo orthopaedic surgery (femoral fracture, tibial fracture, patellar fracture, ankle fracture) under spinal anesthesia were randomized to be administered 12 mg methylnaltrexone or 12 mg of normal saline placebo subcutaneously following the administration of intrathecal morphine. The purpose of this study was to examine the remedial effects of methylnaltrexone on the possible side effects of intrathecal morphine administered after orthopaedic surgery under spinal anesthesia. The findings demonstrated a significant reduction in postoperative pain and nausea/vomiting after intrathecal morphine in the methylnaltrexone group compared to the saline group, but no significant differences in the rate of urinary retention or incidence of pruritus between both groups.