Clenbuterol, a beta-2-adrenoceptor agonist and the only FDA-approved medication for horses with reversible bronchospasm, is commonly used to treat horses with inflammatory airway disease (IAD) and recurrent airway obstruction (RAO, commonly known as heaves). Although it is often regularly administered to racehorses throughout the racing season, the long-term effects of the drug remained unclear.

To that end, a team of researchers at University of Pennsylvania School of Veterinary Medicine’s (PennVet) New Bolton Center carried out a study on the topic and found that while clenbuterol initially reduces airway sensitivity to inhaled histamines, long-term use can result in reduced bronchoprotective effectiveness.

"Many horses, especially race horses, receive clenbuterol every day," explained Rose Nolen-Walston, DVM, assistant professor in the Department of Clinical Sciences at PennVet.

Using a cross-over design, eight retired Thoroughbred racehorses with IAD were treated over a three-week period with either clenbuterol or a placebo. A baseline measurement for airway reactivity was assessed initially, followed by subsequent measurements every seven days