Use of opioids for pain and anesthetic management in horses

Opioids have been used in a wide range of species, but their use in horses has never been straightforward. Any usefulness is restricted because there appears to be a very narrow margin between analgesia and arousal or excitation. Indeed, the analgesic potency of opioids in horses is not readily and consistently quantifiable, and there are a number of other undesirable side effects of opioids that must be considered clinically. In this regard, the action of opioids in horses deviates markedly from the more desirable actions of depressed behavior seen in, for example, humans and dogs. In this chapter we review objective evidence for and against routine use of opioids in managing pain and general anesthesia in horses.