Synergism between tramadol and parecoxib in the orofacial formalin test

The aim of this study was to evaluate the interaction between tramadol and parecoxib in the orofacial formalin test. Tramadol (10, 31.6, 56, and 100 mg/kg ip) or parecoxib (31.6, 56, 100, and 178 mg/kg ip) were administered 10 min before formalin (2.5%) injection into the upper lip to characterize the dose-response curve of each individual drug in the orofacial pain test in mice. Once the dose-response curve of each drug was obtained, an experimental effective dose 50 (ED50 ) value was determined for each drug. The tramadol-parecoxib combination was evaluated in four different groups of animals. The isobolographic analysis and the interaction index were used to evaluate the nature of interaction between both drugs. The isobologram and the interaction index showed increased in the antinociceptive effect of the combination. The tramadol-parecoxib combination produces a synergism in the second phase of the orofacial formalin test.