The drug is both a decongestant and a bronchodilator. A decongestant thins the blood to reduce blood pressure while a bronchodilator widens the vessels that carry oxygen, so the volume of oxygen in the blood increases.
In the U.S. in the past, clenbuterol has been used in animal rearing as well as by vets. In 1991, the U.S. Food Safety and Inspection Service found it had been fed to livestock, so the animals gained more muscle and less fat. But, again, in many countries, clenbuterol is illegal for animal use.
The drug is now controversial because of its use in bodybuilding and weight-loss programs.