Fig. 2. Time–effect curve for postoperative pain scores from extubation time (EXT) until the 24th postoperative hour. Mean visual analog scale scores (± SD) are plotted against time. Pain scores were significantly lower in the morphine group from extubation to 30 min. Two-way repeated measures of analysis of variance (group and time):P = 0.02, F = 4.3, df = 2 for the group factor, protected least significant difference test was used for post hoc analysis;P = 0.03 between the placebo and morphine groups, P = 0.008 between the morphine and M-6-G groups, and no difference was noted between the placebo and M-6-G groups (P = 0.6). From 30 min until the end of the observation period, no statistically significant difference was noted between groups, and pain scores significantly decreased during the study period within each group (time factor):P = 0.0000001, F = 20, df = 9.