Antihistamines and decongestants.

Drug therapy for allergic rhinitis is used either to prevent symptoms from occurring by short-circuiting the reaction and thus inhibiting the production of chemical mediators or to control symptoms after the target organs have been stimulated by these mediators. Antihistamines, the mainstay treatment of allergic rhinitis, are H1-receptor antagonists that bind competitively to histamine receptors. The older, classic antihistamines are effective in treating the symptoms of allergic rhinitis, but most are sedating because they cross the blood-brain barrier. They also have anticholinergic activity, which further restricts their use. The new, nonsedating antihistamines have overcome most of these limitations, and because they are long-acting, they require fewer daily dosages. Many still under development are quite potent and may be used for indications other than allergic rhinitis. Decongestants, sympathomimetic amines, are available both orally and topically as either short- or long-acting preparations. Topical decongestants should be used only for a short time to prevent rebound and ensuring overusage. These drugs interact with numerous antihypertensive medications and tricyclic antidepressants. Often combined with antihistamines, decongestants help offset their sedative effect.