General Medical Information

Antihistamines

Shawn D. Newlands

Oral antihistamines are effective in reducing the symptoms of itching, sneezing, and rhinorrhea in allergic rhinitis. Histamine released from mast cells and basophils dilates blood vessels, increases vascular permeability, and stimulates parasympathetically mediated glandular secretions. These effects are mediated by the H1 histamine receptor. Classic first-generation antihistamines cross the blood-brain barrier and act centrally, which produces sedation. Use of these drugs causes learning impairment among children and contributes to automobile deaths due to central nervous system depression and impairment. Antihistamines also contribute to decreased productivity and work injury.

Second-generation H1 antagonists are nonsedating because they do not enter the central nervous system. Earlier second-generation drugs, including astemizole and terfenadine (withdrawn from the U.S. market in 1998), occasionally caused ventricular arrhythmia, especially when used in excessive doses, in the presence of severe liver disease and in combination with some macrolide antibiotics (erythromycin, clarithromycin, troleandomycin) and azole antifungal agents (ketoconazole, itraconazole). The newer nonsedating antihistamines, such as cetirizine, fexofenadine, and loratadine, are recommended because they do not cause cardiac disturbances. Antihistamines are first-line therapy for allergic rhinitis among children, although none of the nonsedating medicines are approved for children younger than 2 years.

Although the aforementioned medicines are first-line therapy for allergic rhinitis, they have no efficacy in the management of nonallergic rhinitis. Proper diagnosis is essential. Systemic antihistamines are not effective in managing nasal obstruction, but they are effective for allergic conjunctivitis. Intranasal antihistamines relieve nasal congestion and other antihistamine effects, but because of bitter taste, patients often do not tolerate them.