Clenbuterol is a chemical called a beta-2-adrenergic agonist. It has approval in the United States for use in horses with breathing difficulty.

Clenbuterol is both a decongestant and a bronchodilator. A decongestant works by narrowing and constricting blood vessels to reduce congestion. Conversely, a bronchodilator helps relax the muscles around the airway, which in turn opens it up.

In some European and Latin American countries, clenbuterol has approval for human use as an asthma drug. Others have used it to treat chronic obstructive pulmonary disease (COPD). However, in the United States, its use is banned for these purposes.

The drug is also controversial due to its use in bodybuilding and weight-loss programs.