The compounds most commonly studied include: cima-terol, clenbuterol, L-644-969, ractopamine, salbutamol, and zilpaterol (Fig. 1). They are orally active and administered as feed ingredients. They belong to a class of compounds called phenethanolamines. This class of compounds has been used safely and effectively in human medicine for more than 30 years as bronchodilators for asthma treatment, uterine relaxants to treat pregnant women to arrest premature labor, and cardiostimulants to treat cardiac irregularities. The compounds that have been studied for their effects on lean efficiency (Fig. 1) have demonstrated effects on the metabolism of fat and muscle in livestock species. They have no antibiotic activity and, therefore, are not acting as antibiotic growth promoters, nor are they anabolic steroids.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.