Statistical considerations The study will be powered to detect a difference in cumulative 24-hour pethidine consumption. Based on a baseline 24-hour consumption of 410 mg 18 with a standard deviation of 100 16 , a power of 0.8 and a significance level of 0.05, the sample size required to detect a 20% difference in pethidine consumption is 24 subjects per group. This difference would be considered clinically significant. A total of 60 patients will be enrolled to allow for loss of data due to participant drop-out.