Loratadine is an addition to the class of nonsedating antihistamines which includes terfenadine, astemizole, and acrivastine. Loratadine is chemically related to the tricyclic antidepressants. Animal data have shown that insignificant amounts of loratadine enter the brain, and it has a threefold greater affinity for peripheral as compared with central histamine 1-receptors. Loratadine has one main metabolite, descarbethoxyloratadine, which is four times more active than the parent drug. Loratadine reaches peak plasma concentration in 1-2 hours; the metabolite does so in 3-4 hours. Their respective elimination half-lives are about 10 and 20 hours. Onset of action is within 1 hour and duration is at least 24 hours. Once-daily dosing is recommended. Generally, loratadine is as efficacious as existing antihistamines in relieving symptoms of allergic rhinitis, urticaria, and in suppressing wheal formation. Reports of sedation and other adverse reactions are no more frequent than found with placebo. Tachyphylaxis has not been noted in humans, and there is minimal potential for drug interactions based on animal data. Loratadine and terfenadine have comparable therapeutic applications. Both have shown minimal adverse effects, but loratadine has a quicker onset and longer duration of action. These two agents are useful for acute allergic reactions. In contrast, astemizole has an onset of action of several days and is most useful for prophylactic treatment of seasonal allergies.

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