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.