Drug combinations have been used in clinical practice for the main purpose of increasing therapeutic effect efficacy. The aim of this study was to determine the antinociceptive effect of tramadol and caffeine administered separately or in combination, as well as their synergistic interaction. The formalin test was used. Nociceptive behavior was evaluated by flinching response of the formalin-treated paw. Rats were divided into five groups and received tramadol alone (4.9-49.6mg/kg, s.c.), caffeine alone (1-17.8mg/kg, p.o.), or combinations of tramadol (4.9, 8.8, 15.6 and 20.8mg/kg, s.c.) and caffeine (1, 3.16 and 10mg/kg, p.o.). Tramadol showed dose-dependent antinociceptive effect in both phases of the formalin test. Caffeine only presented antinociceptive effect in the second phase and this effect was also dose-dependent. In Phase 1, combinations of tramadol and caffeine showed antinociceptive effect similar to that of tramadol alone. In Phase 2, the dose-response curve shifted to the left with the combination of tramadol and each dose of caffeine. Synergism analysis resulted in synergistic effect in ten combinations and antagonism in two combinations. In conclusion, the synergism observed in the majority of tramadol and caffeine combinations used in this study suggests that this drug combination is useful in the treatment of pain. Copyright © 2010 Elsevier Inc. All rights reserved.