Fig. 4. Effect of intrathecal administration of the drugs studied in the tail-flick test. Time course of the antinociceptive effect of oxycodone ( A ), oxymorphone ( B ), and noroxycodone ( C ) after intrathecal administration in the tail-flick test. The mean of the maximum possible effect (MPE%) ± SEM is plotted over time (in minutes). * Statistically significant differences ( P < 0.05) as compared with the saline control.