Clenbuterol drug, molecular model. This decongestant and bronchodilator is used to treat breathing disorders. It acts on beta-2 adrenergic receptors to relax the smooth muscle of the airways. It has been banned in the UK since 1996 due to side effects that include palpitations and arrhythmia (irregular heartbeat). It also increases fat burning and muscle mass, and is used illegally as a slimming aid, and by athletes as a performance-enhancing drug. The molecular formula is: C12.H18.Cl2.N2.O. Atoms (spheres) are colour-coded: carbon (dark grey), hydrogen (light grey), chlorine (green), nitrogen (blue), and oxygen (red).