Inhaled β2-agonists have been used for > 40 years in the treatment of asthma. Until fairly recently, their regular use had been advocated as the first-line therapy for asthma treatment. This is because inhaled β2-agonists provide rapid bronchodilation as a result of their action as airway smooth muscle relaxants, and, thus, provide rapid improvement of symptoms. Also, inhaled β2-agonists protect against stimuli, such as exercise, allergen, or pollutants, that cause bronchoconstriction in asthmatic patients. For these reasons, inhaled β2-agonists are the most widely prescribed and used drug in the treatment of asthma in many (possibly most) countries, and their regular use is still regarded by many physicians as a first-line treatment option.