Veterinarians in Argentina recently tried to use versions of the topical NSAID (non-steroidal anti-inflammatory drug) diclofenac made for human pain management on horses, but found horses' skin does not absorb those formulations of the drug as well as human skin does. M. Fabiana Landoni, DVM, PhD, of the Universidad Nacional de La Plata, and colleagues, published their findings in a new study.

Topical formulations of diclofenac help to control pain and ease inflammation with fewer side effects than oral or injectable formulations, said Landoni, a past president of the Argentinian Association for Veterinary Pharmacology and Therapeutics.

A diclofenac formulation known as Surpass (manufactured by Boehringer Ingelheim Vetmedica) is approved for use in horses in the United States but is not available in all countries. It uses a liposome enhancer–a fatty substance–to help deliver medication to the body.

In the new study, the researchers tried mixing straight diclofenac with several substances to see if they could enhance the absorption of the drug through the horse's stratum corneum (the outermost layer of the skin). They found that all of the five substances tested improved the drug's absorption, but that a substance called limonene, a chemical compound found in the oil of citrus fruit, worked best and required a lower concentration of diclofenac