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Various opioids are available for use in equine medicine. Studies directly comparing their analgesic effects and side effects are rare. Therefore, the aim of this study was to compare the antinociceptive effect and the duration of analgesia of two different doses of morphine, butorphanol and levomethadone in horses.Eight adult, healthy horses were used for this randomized, placebo-controlled, blinded cross-over trail. Each horse received placebo (P = 0.9% saline) and morphine (MIn group MLevomethadone induced a dose-dependent increase and prolongation of analgesia, whereas with butorphanol there was no difference between dosages regarding duration and intensity of analgesia. Morphine provided detectable analgesia only in the high dose of 0.2 mg/kg.Levomethadone and morphine in the low dose (0.1 mg/kg) produced only minor and short lived anti-nociception and further studies are necessary to give a profound dose recommendation for the use of these drugs in horses.

The combination of Medetomidine and Butorphanol leads to a sufficient sedation with good analgesia and is thus superior to the application of Medetomidine only. By reducing the dosage of Medetomidine, reduced cardio-respiratory side-effects are to be expected. The comparative application of the two Butorphanol preparations Torbugesic (group T) and Morphasol (group M) did not show any differences concerning effect and tolerance. No statistically significant and clinically relevant differences between the two groups were observed for any of the parameters.

In view of the basic principles of the receptor theory, one should expect all opioids to exhibit principally the same spectrum of effects, as they exert their actions by interaction with "the opiate receptor." Clinical experience shows, however, that the opioids in clinical usage in fact exhibit a variety of spectra of effects. This discrepancy is explained by introducing the "multiple receptor theory," as developed by Gilbert and Martin. This review attempts to class practical clinical consequences with those more recent pharmacological findings.