Anti-hyperalgesic effects of tramadol in the rat

A new simple behavioral method was used for the evaluation of the anti-hyperalgesic properties of tramadol in the rat. At the lowest dose (1.25 mg/kg i.p.), tramadol did not modify thermal nociceptive thresholds, but it was able to prevent and block thermal hindpaw hyperalgesia induced by the tail injection of formalin. Our results provide evidence that tramadol blocks hyperalgesic behaviors without altering nociception, suggesting that this analgesic drug might represent a valid agent against central sensitization.