Anything that opens or expands the bronchi (that part of the body that conveys air to and from the lungs). Bronchodilating drugs are usually prescribed if a cough occurs with airway narrowing and can reduce coughing, wheezing, and shortness of breath. Bronchodilators can be taken orally, injected or inhaled and begin to act almost immediately but with the effect only lasting 4-6 hours.
The most common bronchodilators are:

• Beta-adrenergic receptor agonists.
  These are the drugs used most commonly to relieve a sudden asthma attack or to prevent an attack during exercise. This type of bronchodilator stimulates beta-adrenergic receptors to widen the airways. Beta-adrenergic receptor agonists act either on all beta-adrenergic receptors (e.g. adrenaline ) which can cause side effects such as headache, muscle tremors and restlessness. However there also exist drugs of this class that act only on beta2-adrenergic receptors in the lungs, thus causing less side effects.
• Anticholinergic drugs.
  Drugs of this class, such as ipratropium bromide and atropine, block acetlycholine from causing smooth muscle contractions and from producing excess mucus in the bronchi. These drugs futher widen the airways of people who are already taking beta2-adrenergic receptor agonists.
• Theophylline
  Drug used in asthma treatment and prevention.