Clenbuterol is a very widely used drug and has quite a reputation. A good one among athletes and recreational users, and a very bad one among those people who know very little about illegal performance enhancing aids. Its not a steroid. In fact, the only medical use for which Clenbuterol is generally prescribed (and now being less and less prescribed thanks to its illegitimate use) is for obstructions of the air-way. People with chronic breathing disorders like asthma use this as a bronchodilator to make breathing easier. But its only one of the many things that can be achieved with the use of Clenbuterol.
In terms of action this drug is best likened to the now also illegal ephedrine and its legal replacement, ma huang. All of them operate mainly by increasing the manufacture and secretion of catabolic hormones known as cathecholamines (like dopamine, epinephrine (adrenaline) and norepinephrine (noradrenaline)) which are secreted from the adrenal region. Now these hormones have a wide variety of functions. First of all they seem to alter the contractile characteristics of smooth muscle, but very specifically. Some will apparently be stimulated, and others inhibited. Amongst those inhibited, the smooth muscles in the bronchial tree, which explains its soothing effect primobolan in patients with breathing problems. What it also does is increase thermogenisis. This usually encompasses a rise in blood pressure, a stimulatory effect of the heart muscle and a resulting rise in body temperature.
Along with the reversing of the effects of insulin (and inhibiting the action of insulin) which results in a release of glycogen back into the blood stream as glucose and an inability to store or use more glycogen, it will increase the rate of protein and fat being burned in the body. For bodybuilders that appears to be the primary use of the drug. This thermogenisis and an increase in the rate of fat being burned usually has as a result that the metabolic rate of the subject its much higher and he burns more calories.