With any medication comes a risk of side effects. For instance, long-term phenylbutazone administration to treat a musculoskeletal issue can result in gastrointestinal problems; pergolide to treat Cushing's disease can cause a decreased appetite; and vaccine administration to protect against disease can cause injection site swelling and muscle soreness. And recently, a research team took a closer look at clenbuterol, a drug used to treat bronchospasm in recurrent airway obstruction (RAO, or heaves), and more clearly defined its side effects on horses' body fat percentage.

At the 2014 American College of Veterinary Internal Medicine Forum, held June 4-7 in Nashville, Tennessee, Rose Nolen-Walston, DVM, Dipl. ACVIM, shared study results on some of clenbuterol's side effects in a poster presentation. The team tested whether long-term, low-dose clenbuterol use would result in decreased body fat, and if exercise affected the magnitude of that change in body fat.

Nolen-Walston, an assistant professor at the University of Pennsylvania School of Veterinary Medicine's New Bolton Center, and colleagues employed eight resting Thoroughbreds and 47 Thoroughbreds at two polo barns to test their hypothesis. The resting horses were separated into two groups and received either 0.8 micrograms per kilogram body weight clenbuterol or corn syrup (as a control) every 12 hours for 21 days before a 7-week washout period. Then, the horses switched treatment groups.

The working horses were also separated into to two groups, one receiving clenbuterol and the other receiving corn syrup in the same manner as the resting horses. Then, the horses had a 21-day washout period before switching treatment groups