Based on a previous study,7 the mean postoperative cumulative morphine consumption at 24 h after caesarean delivery was 38 mg with a standard deviation of 14 mg. An a priori power analysis indicated that 45 subjects were required in each group to detect a 25% reduction in the cumulative morphine consumption at 24 h after caesarean delivery, that was assumed to have a clinically significant effect, with a type-I error of 0.0167 (0.05/3 possible comparisons) and a power of 90%. Additional patients (10%) were included for a final sample size of 150 patients to account for patient dropout during the course of the study.