Morphine is a useful analgesic (painkilling drug) for use in horses, but it’s also known to cause allergic reaction via histamine release – a biological reaction triggered by allergens – in dogs and other species. To learn whether morphine has a similar effect in horses, researchers from the Western College of Veterinary Medicine (WCVM) recently completed a study comparing histamine release levels between morphine and butorphanol (another type
of opioid analgesic) during anesthesia. While morphine is known to cause histamine releases in other species, butorphanol does not.

However, the researchers’ results indicated that histamine concentrations were similar between the two drugs, which means there may be other factors contributing to histamine release. As a continuation of a previous study, the WCVM team will measure the histamine concentration in horses about to undergo surgery in three different scenarios: after no medication (the study’s baseline), following routine doses of penicillin, and after doses of guaifenesin – another potential histamine-releasing drug used in anesthesia.

Findings from this study will help researchers better understand the results of their previous work. Depending on the results, it may also alert anesthesia teams of other sources of histamine release that could affect equine patients.