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.
One serious concern is postoperative colic in horses that have been treated opioids, as these medications are known to cause constipation. Again, past studies are inconsistent in their reporting of the drugs' effects in horses. Clutton said there are other factors that can contribute to colic in horses after surgery, including diet and stress. He added that inadequately treated pain can actually increase the likelihood the horse will develop colic.
Other reported concerns include the effects of these medications on the heart and lungs, as opioids have been linked to low blood pressure and decreased respiratory rates in people. Clutton said that these side effects have the potential to appear in horses if high doses of intravenous opioids are given too quickly, resulting in a shock to the system.
According to Clutton, the recommended doses of opioid drugs vary widely and depend on many factors. They should not be administered on a set time schedule, but, rather, based on the desired level of analgesia, he said.
"The use of opioid analgesics in horses can be justified when the benefits of their analgesic and sedative properties outweigh the disadvantage of potential side effects," he wrote.
Clutton cautioned that horse owners should discuss the potential effects with their veterinarians and report any apparent side effects immediately.
The review, "Opioid analgesia in horses," was published in the December issue of Veterinary Clinics of North America. The abstract is available on PubMed.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.