Figure 5. An isobologram of antinociceptive median effective dose (ED50) values and 95% confidence intervals (CIs) for morphine (horizontal), lidocaine (vertical) or a combination of morphine and lidocaine (point in the dose field). The heavy lines represent the CIs. The dashed diagonal line connecting the morphine and lidocaine ED50values (closed square on the axes) are the theoretical additive line, and the point on this line (large closed square) is the theoretical additive point. The fact that the experimental points (closed circle) evaluated fell below the theoretical additive points indicates that the antinociceptive effects produced by the combination were synergistic.