Fig. 3. Isobologram showing the peripheral interaction of lamotrigine and morphine in the formalin test. Horizontal and vertical bars indicate SEM. The oblique line between the x- and y-axes is the theoretical additive line. The point in the middle of this line is the theoretical additive point calculated from the separate ED³⁰values. The experimental point lies far below the additive line, indicating a significant synergism (P = 0.016).