Fig. 3. Dose–response relationships for pregabalin (open diamond, solid line), naproxen (open square, solid line), and the combination of the pregabalin and naproxen (filled-in circle, solid line). The slope of the dose–response curve for pregabalin is significantly steeper than that of naproxen. Therefore, the theoretical dose–additive line (dashed line) for the total dose of pregabalin and naproxen at each fixed-dose combination was constructed as described by Tallarida 19(A ) The dose of the 10:1 pregabalin:naproxen (P:N) combination required to achieve the specified level of effect (i.e ., an increase in PWL to 7.0 s) is significantly less than the theoretical dose–additive point (P < 0.05). In contrast, the doses of the 1:1 pregabalin:naproxen combination (B ) or the 1:10 pregabalin:naproxen combination required to increase PWL to 7.0 s do not differ from their respective theoretical dose–additive points (P > 0.05, both combinations). Symbols represent the mean ± SEM response latencies from 7–16 rats.