Clenbuterol is a β2-adrenergic agonist that is used in horses as a short-term bronchodilator for the management of airway obstruction and for chronic breathing ailments in horses such as Chronic Obstructive Pulmonary Disease (COPD, also known as "heaves"). Clenbuterol’s action as a bronchodilator means that it relaxes the smooth muscle tissue that makes up the airways, allowing for freer breathing.
The United States Food and Drug Administration (FDA) has approved clenbuterol only for use by veterinarians, who may prescribe it to treat horses with the maladies described above. It is also occasionally used as a uterine relaxant for dystocia.1 “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.”2
There are a variety of proprietary (brand) names for clenbuterol, which is typically offered as an orally-administered syrup. Originally, clenbuterol was given to livestock to increase lean muscle mass and livestock production, but was banned for this purpose due to toxicity issues rising in humans who consumed meat products that were tainted with residual amounts of the drug.
Clenbuterol’s muscle-building properties are tied to its ability to increase the development of skeletal muscle by enhancing muscle protein synthesis.3 Aging decreases skeletal muscle mass and strength, which may be exacerbated by age-related diseases. Clenbuterol dramatically increases skeletal muscle mass in young animals and humans, and partially prevents or restores muscle loss in experimental models of muscle wasting.4 Clenbuterol isn’t a steroid, but it has some properties similar to those of anabolic steroids, such as promoting an increase in muscle mass.
In addition to its ability to increase muscle mass and reduce body fat, clenbuterol remains in the body with an active effect for up to about six days after consumption, while detectable traces can remain longer.5 Because of these properties, clenbuterol has become popular as a weight loss supplement and to enhance athletic performance in humans. Individuals taking clenbuterol for weight loss or performance enhancement often use anabolic steroids or growth hormones as well.
Studies on the efficacy of clenbuterol as a weight loss or performance enhancer in humans are quite limited, although many studies have been performed in animals and livestock:
Despite the fact that there is minimal evidence for clenbuterol as a performance-enhancing drug, it’s listed on the World Anti-Doping Agency’s (WADA) Prohibited List.5
Athletes who use clenbuterol do it to burn fat, build muscle, and improve sports performance, despite the dangers to their health and the possibility that they might be disqualified from sporting events or even banned from their sport for testing positive for performance-enhancing drugs. In fact, athletes who test positive for clenbuterol often claim they must have eaten contaminated meat.5
Clenbuterol can have negative side effects when overused in horses or misused by humans. These can include increased heart rate, rapid breathing, heart palpitations, chest pain, tremors, anxiety and electrolyte imbalance. In horses, side effects are unlikely with clenbuterol, but may include increased heart rate, trembling, excitement, and sweating. It should not be used in horses with heart problems because of the possibility of increased heart rate.
For bronchospasm in equines, the recommended dosage via oral route is typically 0.0008 mg/kg at a concentration of 0.0725 mg/ml, 2 times a day for 3 days. The dosage is increased by ~8mg/kg every 3 days until the horse shows improvement, with a maximum of 30 total days of treatment.
Clenbuterol is FDA-approved for use in horses only and is a prescription-only drug. U.S. federal law restricts this drug to use by or on the written or oral order of a licensed veterinarian. Clenbuterol is prohibited or regulated in most sanctioned equine competitions.