Cross-tolerance between morphine and ethanol and their antinociceptive effects

Morphinization of mice (37.5 mg morphine) and rats (75 mg) with subcutaneously implanted pellets has resulted in tolerance to the antinociceptive effect of morphine, measured in the tail-immersion test. All animals also developed cross-tolerance to the analgesic effects of ethanol (2.8 g/kg intraperitoneally for mice, per os for rats). These results suggest a commonality between morphine and ethanol antinociceptive action.