Clenbuterol is a type of medication that's a selective beta-2 agonist/antagonist and bronchodilator. That means it relaxes the smooth muscle tissue that makes up the airways to allow for freer breathing.
 
The United States Food and Drug Administration has approved clenbuterol only for use by veterinarians, who may prescribe it to treat horses with obstructive pulmonary disease or other lung conditions.2 The trade name for clenbuterol used for horses is Ventipulmin.
 
The "human equivalent" of clenbuterol is albuterol, which is a prescription-only medication used to treat or prevent bronchospasm caused asthma, bronchitis, emphysema, and other lung diseases, and also to prevent wheezing caused by exercise.
There are a variety of brand names for albuterol, including Ventolin, Proventil, Accuneb, among others. Regardless of whether it comes in generic or brand name form, albuterol comes in an inhaler that a person can use to breathe the medication in through the mouth so it can easily reach the airways.
 
 
Clenbuterol was previously given to livestock to increase lean muscle mass and livestock production. Now it's banned from being used for this purpose.3 Athletes who test positive for clenbuterol often claim they must have eaten contaminated meat. Three-time Tour de France winner Alberto Contador may be the most famous athlete to have used this excuse after testing positive for clenbuterol.
 
Athletes who use clenbuterol do it to burn fat, build muscle, and improve sports performance. The drug is believed to increase the development of skeletal muscle by enhancing muscle protein synthesis.4 At the same time, it aids in fat loss by increasing metabolism.
 
In general, any positive effects of taking clenbuterol seem to be temporary and short-lived.
Risks and Side Effects 
The effects of clenbuterol on athletes who take the drug over a long period of time aren't known. What is clear is that besides affecting the muscles of the airways, clenbuterol is taken up by other tissues in the body.
 
Animal studies have shown that clenbuterol use may lead to apoptosis—the death of normal cells—in the muscles, including the heart muscles.5 For this reason, veterinarians are cautioned against giving the drug to horses who have cardiac issues.
 
Clenbuterol may lead to heart attacks and other heart damage, as well as irregular, heart rhythms.6 Other side effects of clenbuterol use include muscle tremors, increased perspiration, increased blood pressure, insomnia, headache, nausea, and vomiting.
Athletes who chooses to rely on a banned performance-enhancing drug are risking being caught for cheating. They also may be risking their health.