Oral beta 2-agonists (carbuterol, pirbuterol, procaterol, bitolterol, clenbuterol) are drugs widely used as bronchodilators. The efficacy and selectivity of bronchodilators drugs depend on their intrinsic pharmacological properties and on the route of administration. The characteristics of the oral route are easy usage, precise dosage and assured effects. Consequently, disadvantages (delayed onset of action, more frequent side-effects) and the indications, (patients with severe chronic airways obstruction, nocturnal asthmatic attacks, and children and elderly subjects) are clearly evident. The most recent beta 2-agonists have an efficient and prolonged bronchodilating action with well-known side-effects. In order to control drug efficiency in a large population and identify type and degree of adverse reactions, a post-marketing surveillance study was programmed for clenbuterol. The results available confirms that long-term treatment with oral clenbuterol is an effective and safe therapeutical approach. During long-term treatment, tachyphylaxis (a diminished responsiveness) develops. This complex biological phenomenon can be studied, in several ways i.e. functional response of target-organ, appropriate biochemical-metabolical indices, and functional evaluation of the cellular beta-receptors in vitro. Also in the light of evaluation of serum levels of cyclic nucleotides (cyclic adenosine and guanosine monophosphates) it appears that the clinical importance of tachyphylaxis is mild and that chronic therapy with beta 2-agonists is safe and effective when used in selected patients.