A bronchodilator or broncholytic (although the latter occasionally includes secretory inhibition as well) is a substance that dilates the bronchi and bronchioles, decreasing resistance in the respiratory airway and increasing airflow to the lungs. Bronchodilators may be endogenous (originating naturally within the body), or they may be medications administered for the treatment of breathing difficulties. They are most useful in obstructive lung diseases, of which asthma and chronic obstructive pulmonary disease are the most common conditions. Although this remains somewhat controversial, they might be useful in bronchiolitis and bronchiectasis. They are often prescribed but of unproven significance in restrictive lung diseases.

Bronchodilators are either short-acting or long-acting. Short-acting medications provide quick or "rescue" relief from acute bronchoconstriction. Long-acting bronchodilators help to control and prevent symptoms. The three types of prescription bronchodilating drugs are β2("beta two")-adrenergic agonists (short- and long-acting), anticholinergics (short- and long-acting), and theophylline (long-acting).

Short-acting β2-adrenergic agonists

These are quick-relief or "rescue" medications that provide quick, temporary relief from asthma symptoms or flare-ups. These medications usually take effect within 20 minutes or less, and can last from four to six hours. These inhaled medications are best for treating sudden and severe or new asthma symptoms. Taken 15 to 20 minutes ahead of time, these medications can also prevent asthma symptoms triggered by exercise or exposure to cold air. Some short-acting β-agonists, such as salbutamol, are specific to the lungs; they are called β2-adrenergic agonists and can relieve bronchospasms without unwanted cardiac side effects of nonspecific β-agonists (for example, ephedrine or epinephrine). Patients who regularly or frequently need to take a short-acting β2-adrenergic agonist should consult their doctor, as such usage indicates uncontrolled asthma, and their routine medications may need adjustment.

Long-acting β2-adrenergic agonists

These are long-term medications taken routinely in order to control and prevent bronchoconstriction. They are not intended for fast relief. These medications may take longer to begin working, but relieve airway constriction for up to 12 hours. Commonly taken twice a day with an anti-inflammatory medication, they maintain open airways and prevent asthma symptoms, particularly at night.

Some examples of anticholinergics are tiotropium (Spiriva) and ipratropium bromide.

Tiotropium is a long-acting, 24-hour, anticholinergic bronchodilator used in the management of chronic obstructive pulmonary disease (COPD).

Only available as an inhalant, ipratropium bromide is used in the treatment of asthma and COPD. As a short-acting anticholinergic, it improves lung function and reduces the risk of exacerbation in people with symptomatic asthma. However, it will not stop an asthma attack already in progress. Because it has no effect on asthma symptoms when used alone, it is most often paired with a short-acting β2-adrenergic agonist. While it is considered a relief or rescue medication, it can take a full hour to begin working. For this reason, it plays a secondary role in acute asthma treatment. Dry throat is the most common side effect. If the medication gets in contact with the eyes, it may cause blurred vision for a brief time.

The use of anticholinergics in combination with short-acting β2-adrenergic agonists has been shown to reduce hospital admissions in children and adults with acute asthma exacerbations.

Common bronchodilators

The bronchodilators are divided into short- and long-acting groups. Short-acting bronchodilators are used for relief of bronchoconstriction, while long-acting bronchodilators are predominantly used for prevention.

Experimental bronchodilator

From among 6000 compounds which relax the smooth muscle cells of the lungs' airways and open up the airways in asthmatic lung tissues, researchers have identified a drug called TSG12. TSG12 is a specific transgelin-2 (TG2)-agonist that relaxes airway smooth muscle cells and reduces asthmatic pulmonary resistance. The authors claim that "TSG12 is both non-toxic and more effective in reducing pulmonary resistance than current bronchodilators and could be a promising therapeutic approach for treating asthma without losing effectiveness over time."

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