Clenbuterol belongs to so-called beta-2-agonists. It stimulates the beta-2-receptors of the sympathetic nervous system whereby the smooth muscle tissues relax in bronchial tubes and the womb, among others.
Beta-2-agonists are used as asthma drugs and to prevent premature childbirth. Clenbuterol is administered by mouth. It is used mainly in veterinary medicine, for example, for the treatment for asthma in horses. Clenbuterol is not used to treat human asthma.
Several beta-2-antagonists, especially clenbuterol, have anabolic (tissue growth-stimulating) effects. They increase the protein content of striated muscles when the glycogen of muscles and body fat burn at the same time. Therefore, clenbuterol is one of the most common substances that are used for the growth of muscle mass and burning fat.
However, it is not classified as a doping substance according to the Decree 705/2002 that defines the doping substances that are to be regarded as doping substances referred to in Chapter 44, §16, Subsection 1 of the Penal Code.
The adverse effects of clenbuterol include, among others, tremors, rapid heartbeat, arrhythmias, muscle cramps and electrolyte disturbances [1, 2, 3]. Arrhythmias and hypokalemia (low levels of potassium in the blood) can lead to death if they are not treated.
The Finnish Antidoping Agency FINADA
 Bilkoo, Thomas, Riddle & Kagaoan (2007): Clenbuterol toxicity: an emerging epidemic. A case report and review. Connecticut Medicine 71(2): 89–91.
 Capretti, Dantes, De Gasperi & D'Ambrosio (1987): Changes in blood potassium induced by the abuse of 3 beta 2-agonist drugs. Clinica Terapeutica 15;121(1): 41–4.
 Daubert, Mabasa, Leung & Aaron (2007): Acute clenbuterol overdose resulting in supraventricular tachycardia and atrial fibrillation. Journal of Medical Toxicology 3(2): 56–60.