Anticholinergic therapy is directed toward muscarinic receptors within the lung. Inhaled quaternary muscarinic antagonists are used because they cause less systemic adverse effects than other types of antimuscarinic drugs. Of the 5 possible muscarinic receptors, the lung contains three: M1, M2, and M3. Antagonism of the M1 and M3 receptors results in bronchodilation, primarily in the larger airways. The efficacy of the antimuscarinic bronchodilators cannot be entirely explained by this mechanism, and there is probably peripheral activity as well. The use of inhaled antimuscarinic drugs is well tolerated, and the efficacy is maintained with chronic use. Although ipratropium is the first-line therapy for symptomatic COPD, it and other similar drugs can also be used to treat other obstructive diseases. Ipratropium is available in an MDI, in solution for small-volume nebulizer, and in combination with albuterol in an MDI. Newer antimuscarinic agents are being developed that are more selective for the M1 and M3 sites and last longer. Research is continuing to better understand the sites of action of these agents as well as to develop even more effective drugs.