Local versus systemic effects of inhaled drugs.

For all drugs, the therapeutic ratio, i.e., the difference between therapeutic effect and adverse effect, should be as large as possible. When focusing on drugs used in the treatment of asthma, particularly beta 2-agonists and corticosteroids, the therapeutic ratio of these drugs can be considered to be the degree of separation of their effects on lung function from their systemic effects. This is increased by administration via inhalation, resulting in targeting of the drug directly to the site of action. However, all drugs deposited in the airways are eliminated via the systemic circulation, and have, therefore, the potential to cause some systemic side effects.