Synergism between the analgesic actions of morphine and pentazocine

In a double-blind, placebo-controlled study the analgesic efficacy of the combination of morphine, a mu-opiate receptor agonist and pentazocine, a kappa-opiate receptor partial-agonist was evaluated. Groups of 20 patients received either vehicle, morphine (2, 4, 8 or 16 mg), pentazocine (15, 30 or 60 mg) or a combination of morphine and pentazocine (2 mg or 4 mg and 15 mg or 30 mg, respectively). The combination of morphine and pentazocine produced a level of analgesia significantly greater than can be accounted for by simple addition of the analgesic effects of each opiate analgesic alone. We propose that the observed synergism between morphine and pentazocine is due to interaction between mu- and kappa-opioidergic components of endogenous opioid-mediated analgesia systems.