Updated December 16, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What Is Clenbuterol?

Clenbuterol, also called "clen," is considered a performance-enhancing drug and is banned in most athletic competitions. Clenbuterol is a selective beta-2 agonist/antagonist and a bronchodilator initially prescribed for obstructive pulmonary disease and asthma because it relaxes smooth muscle. Although clenbuterol is not approved for human use in the U.S., it is virtually identical to albuterol, which is often prescribed as a treatment for asthma.

In the U.S., clenbuterol is only FDA-approved when prescribed by a veterinarian for use in horses.
Although clenbuterol use in livestock is now banned in the U.S. and much of Europe, it has been used to increase the lean mass and the overall yield of livestock production. Because of this, some 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 claim his positive clen test was due to eating meat from an animal treated with clenbuterol.

Clenbuterol Use in Sports

The World Anti-Doping Agency and the International Olympic Committee include clenbuterol on their lists of performance-enhancing drugs, and prohibit athletes from using it. Many athletes continue using clenbuterol to burn fat, build muscle and improve sports performance.
Even though clenbuterol use is banned in most organized sports competitions, many athletes are still found using the substance. Clenbuterol is believed to increase the development of skeletal muscle by enhancing muscle protein synthesis, while at the same time aiding fat loss by increasing metabolism.

Clenbuterol is also taken up by other tissues in the body. Animal studies have shown that clenbuterol use may lead to cell death or apoptosis in the muscles, including the heart. In general, the positive effects of clenbuterol seem to be short-lived. The effects of long-term clenbuterol intake on an athlete's performance, muscle strength and overall heart health are still unclear.

Clenbuterol Side Effects

Clenbuterol may cause cardiac hypertrophy (enlargement of heart ventricles), a potentially serious form of heart damage, as well as irregular rhythms, and as stated above, specific cell death. Other side effects of clen use may include muscle tremors, increased perspiration, increased blood pressure, insomnia, headache, nausea and vomiting.
Athletes who are considering using clenbuterol need to seriously weigh the risks and benefits of this drug. Both the long-term and short-term side effects appear to be quite dangerous. Athletes using clen also need to be aware that they can't be assured of the safety, purity or quality of the product they end up using. If your goals are to gain muscle and lose fat, there are safer, legal ways to achieve that outcome, such as a well-designed nutrition and training routine.

<sub>Sources:

The National Institute on Drug Abuse, www.drugabuse.gov, Steroid Abuse and Addiction, August 2006.

The World Anti-Doping Agency Prohibited List, [http://www.wada-ama.org/en/World-Anti-Doping-Program/Sports-and-Anti-Doping-Organizations/International-Standards/Prohibited-List/], last accessed November 2010.</sub>