Prospects for antihistamines in the treatment of asthma.

Antihistamines have been evaluated as potential therapies for asthma for more than 50 years. With first-generation compounds, side effects prevented effective dosing. By reviewing published studies of the effectiveness of terfenadine, cetirizine, and loratadine in clinical asthma, evidence for the relevant effect of the second-generation antihistamines on bronchial asthma can be found. Terfenadine, at doses of 120 or 180 mg twice a day, reduced symptoms and improved pulmonary function in mild and moderate allergic asthma but was ineffective in severe perennial asthma. Fexofenadine at doses used for allergic rhinitis had little effect on seasonal allergic asthma. Research is ongoing to determine the effects of higher doses of fexofenadine. In 5 studies, cetirizine at doses of 10 to 20 mg, once or twice daily, consistently improved asthma symptoms compared with placebo or terfenadine 60 mg twice a day in 2 cases, whereas in 2 studies, loratadine at doses of 10 to 20 mg daily has not produced significant improvement in asthma. However, loratadine 5 mg combined with 60 mg of pseudo-ephedrine twice a day significantly improved both asthma symptoms and peak expiratory flow. Similarly, the combination of loratadine 20 mg and the leukotriene-receptor antagonist montelukast improved asthma symptoms, peak expiratory flow, and beta-agonist use over montelukast alone. Therefore, there might be a role for second- and third-generation antihistamines in treating mild and moderate asthma, which might require administering doses greater than those commonly used to treat allergic rhinitis. If higher doses are sedative, the addition of decongestants or leukotriene-receptor antagonists might enhance the effects of lower doses of the antihistamines.