Despite the fact that opioid drugs, such as morphine and methadone, have been known to cause serious side effects in some horses, a researcher from Scotland maintains that they are a good choice for treating severe pain in horses.

R. Eddie Clutton, BVSc, MRCVS, DVA, MRCA, a professor of anesthesia at the University of Edinburgh, wrote a comprehensive review on the use of these medications in equines, in which he noted that many concerns about side effects are based on very early and less rigorous studies than those done today. Newer research shows a "widening role for opioid analgesics," he said.

These narcotic medications act on the central nervous system to decrease pain perception and are often chosen for cases with severe pain.

Side effects–such as excitability and sensory and motor depression–observed in early studies often resulted because researchers gave much higher doses than those given today, explained Clutton. To complicate matters, the term "excitation" when applied to opioid-induced side effects in horses described a wide range of activities from head nodding to muscle twitching, and it was difficult to know exactly what effects the opioids caused, he said