Fig. 5. Isobologram of the interaction between intrathecal gabapentin and clonidine. The ED50values for each agent are plotted on the x -axis and y -axis, respectively, and the thick lines present the 95% confidence intervals of each ED50. The dotted line connecting these two points is the theoretical line of additivity. The theoretical additive ED50point (A) and 95% confidence interval are calculated from the ED50values and 95% confidence intervals of each agent. The experimentally observed ED50point (B) with 95% confidence interval for the combinations lies outside the region defined by the theoretical line of additivity. The confidence intervals of A and B do not overlap, indicating a synergistic interaction.