Clenbuterol hydrochloride is a β2-adrenergic agonist that is used in horses as a short-term bronchodilator. In the short-term, it is typically employed in the management of airway obstruction (eg, severe equine asthma [SEA], formerly known as recurrent airway obstruction [RAO]).1 It is also used in the treatment of Chronic obstructive pulmonary disease (COPD, also known as "heaves"). Clenbuterol is also occasionally used as a uterine relaxant for dystocia in equines. Clenbuterol works by relaxing the smooth muscles surrounding the airways and opening the passages, and also serves to loosen excess mucus.
In May of 1998, the U.S. Food and Drug Administration (FDA) approved a branded variety of clenbuterol syrup for use in horses. No other uses for clenbuterol were approved; all other uses in horses or other animals are extra-label, and the drug was not approved for use in humans. In fact, due to toxic side effects, clenbuterol, which was previously given to livestock to increase lean muscle mass and livestock production, is now banned in the U.S. for use in food animals.
Despite the legal and toxicity issues connected with its use outside veterinary medicine, clenbuterol has become popular among those who wish to lose weight, and among athletes, particularly those involved in professional bodybuilding.
Initially, manufacturers of clenbuterol for equine use took steps to ensure that the approved drug would not be misused. These included instructing salespersons to notify practicing veterinarians that its use in food animals was prohibited, notifying salespersons and veterinarians that law enforcement agencies have the authority to test food animals for signs of exposure to clenbuterol and providing sales information on clenbuterol to the FDA, including the names and addresses of purchasers (to facilitate the agency’s ability to track suspicious distribution patterns).3
Clenbuterol (which is also called "clen" in the athletic community) is considered a performance-enhancing drug and has been banned from most athletic competitions. Both the World Anti-Doping Agency and the International Olympic Committee include clenbuterol on their lists of prohibited drugs.2 Despite these bans, athletes continue using clenbuterol for its ability to help burn fat, build muscle, and improve performance.
Since clenbuterol has some properties similar to anabolic steroids, it is claimed to increase muscle mass. Because of this, bodybuilders and athletes sometimes take the oral or injectable forms. It has also been reported to reduce body fat, making it a popular weight loss supplement. Unfortunately, studies that support these claims of desirable effects on muscle and fat have only involved livestock and experimental animals such as mice and rats. Studies in humans have not shown similar benefits.4
Pharmacokinetically, the mechanism in clenbuterol use (among human athletes) is in improving the thermogenesis that involves a small boost in hypertension which stimulates the muscle included in the heart, thus bringing about rising from the body’s temperature. It also increases the speed in output of protein as well as the burning from the fat via the body which results in a higher metabolic process ultimately causing burning of calories, thereby resulting in the shedding of fat.
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.
The risks of misusing a drug like clenbuterol include its lack of proven effectiveness and its hazardous side effects. Clenbuterol is known to cause symptoms such as rapid heart rate (tachycardia), palpitations, tremors, anxiety, lowered blood potassium (hypokalemia), and elevated blood sugar (hyperglycemia). Adverse effects are more likely to occur with the large doses used for performance enhancement and weight loss, which often run about 5 to10 times higher than the regular adult doses of albuterol (the FDA-approved bronchodilator for humans).
Because clenbuterol has a long half-life in the body, toxic symptoms can last from 1 to 8 days. In published reports, over 80% of people who developed toxic effects required care in a hospital.5 The medical literature contains numerous reports of patients with symptoms ranging from agitation and rapid heart rate all the way to seizures and cardiac arrest after swallowing abnormally large clenbuterol doses.
Unlike anabolic steroids, clenbuterol is not a controlled substance.2 However, clenbuterol has been identified as an adulterant in street drugs such as heroin. After snorting or injecting heroin contaminated with clenbuterol, some patients have reported painful muscle spasms, agitation, and overactive reflexes. In 2005, the Centers for Disease Control and Prevention (CDC) reported 26 cases of clenbuterol poisoning among heroin users. They were hospitalized with symptoms such as rapid heart rate, palpitations, hypokalemia, chest pain, and agitation. Most were kept in the hospital for 5 days where they received intravenous fluids, potassium, and medications to slow down the heart. It is likely that many such cases go unreported because patients are afraid to seek medical care or health care providers might not recognize the symptoms of clenbuterol poisoning in illicit drug users.4
1Plumb’s Veterinary Drugs.