Effects of ephedrine and phenylpropanolamine on the antinociceptive effects of morphine and codeine in mice.

The effects of ephedrine and phenylpropanolamine on the antinociceptive activities of morphine and codeine were investigated. Both morphine and codeine exhibited dose-dependent antinociceptive activities in the tail flick test. Ephedrine (5, 10 and 20 mg/kg) and phenylpropanolamine (5, 10 and 20 mg/kg) showed no antinociceptive effect when administered alone. The antinociceptive effects of morphine were enhanced in mice pretreated with ephedrine or phenylpropanolamine. similarly, codeine antinociception was increased in mice pretreated with ephedrine or phenylpropanolamine. In all cases, the ED50 values and single dose comparisons were shifted in the same direction. These effects on the antinociceptive potencies of morphine and codeine were found to be dose-dependent, being statistically significant at the higher dose levels of ephedrine and phenylpropanolamine used in the present study.