Background information at  https://thehorse.com/ViewArticle.aspx?ID=947 

From the Food and Drug Administration (FDA)

In 1998, the FDA approved a new animal drug application (NADA) for Boehringer-Ingelheim Vetmedica’s Ventipulmin Syrup, which contains a small amount of clenbuterol, as a restricted use prescription-only drug for treating horses affected with airway obstruction. When FDA approved the NADA for Ventipulmin, several controls were put in place to ensure that this drug would not be misused in food-producing animals.

Ventipulmin is the only clenbuterol drug product approved for use in the United States. Ventipulmin may only be used in horses not intended for food. FDA has special concern with clenbuterol, a beta-agonist drug that has been used illegally in the U.S. to enhance production of food animals. The use of clenbuterol in other countries has resulted in documented adverse reactions in humans who ingested meat-containing residues of clenbuterol