Clenbuterol is a widely used bronchodilator in many parts of the world. The drug is most often prepared in 20mcg tablets, but it is also available in syrup and injectable form. Clenbuterol belongs to a broad group of drugs knows as sympathomimetics. These drugs affect that sympathetic nervous system in a wide number of ways, largely mediated by the distribution of adrenoceptors. There are actually nine different types of these receptors in the body, which are classified as either alpha or beta and further subcategorized by type number. Depending on the specific affinities of these agents for the various receptors, they can potentially be used in the treatment of conditions such as asthma, hypertension, cardiovascular shock, arrhythmias, migraine headaches and anaphylactic shock. The text Goodman and Gillman's The Pharmacological Basis of Therapeutics Edition does a good job of describing the diverse nature in which these drugs affect the body:

`Most of the actions of catecholamines and sympathomimetic agents can be classified into seven broad types: (1) peripheral excitatory action on certain types of smooth muscles such as those in blood vessels supplying the skin, kidney, and mucous membranes, and on the gland cells, such as those of the salivary and sweat glands; (2) a peripheral inhibitory action on certain other types of smooth muscle, such as those in the wall of the gut, in the bronchial tree, and in blood vessels supplying skeletal muscle; (3) a cardiac excitatory action, responsible for in increase in heart rate and force of contraction; (4) metabolic actions, such as an increase in the rate of glycogenolysis in liver and muscle and liberation of free fatty acids from adipose tissue; (5) endocrine actions, such as modulation of the secretion of insulin, rennin, and pituitary hormones; (6) CNS actions, such as respiratory stimulation and, with some of the drugs, an increase in wakefulness and psychomotor activity and a reduction in appetite; and (7) presynaptic actions that result in either inhibition or facilitation of the release of the neurotransmitters such as such as norepinephrine and acetylcholine."

Clenbuterol (Clen) is a selective beta-2 agonist/antagonist and a bronchodilator. What this means, is that it stimulates your beta-2 receptors. Of great importance, is that Clenbuterol is a selective beta-2 agonist (because it works selectively on the beta-2-andrenergic-receptors), right? The thing is, Clenbuterol is selective...like hitting a tack (the tack being your beta-2 receptors) with a small hammer (the hammer being the Clen)...thus, it hits the beta-2 receptors selectively. Sorry if that seems repetitious, but it´s very important to understand that fact before we move on. Since clenbuterol has very little beta-1 stimulating ability, it has the ability to reduce certain kinds of airway obstruction without much in the way of cardiovascular effect (more about that later), and this is why it is used as an asthma medication.

So what exactly dose a stimulant like Clen (or Ephedrine) do when it stimulates those Beta Receptors? Well, it serves to increase your body temperature a bit by increasing heat production in the Mitochondria, increase your basal metabolic rate, and decrease your appetite (1). This partly explains how Beta-2 agonists directly stimulate fat cells and increase lypolysis (fat-loss)(1)(13). And also, because it is a Beta-2 agent, clen can decrease insulin sensitivity (2), unfortunately.