Cetirizine is a metabolite of hydroxyzine, levocetirizine is an enantiomer of cetirizine, desloratadine is a metabolite of loratadine, and fexofenadine is a metabolite of terfenadine.
**In the United States, these H antihistamines azelastine, emedastine, epinastine, ketotifen, levocabastine, and olopatadine are available in ophthalmic formulations; and azelastine, dimethindene, levocabastine, and olopatadine are available in intranasal formulations.
antihistamine formulations: onset and duration of action is based on usual adult dose of 1 to 2 sprays in each nostril or 1 drop in each eye.
n/a indicates information not available or incomplete.
Second-generation H antihistamines such as cetirizine, desloratadine, fexofenadine, levocetirizine, and loratadine cross the blood-brain barrier to a significantly smaller extent than their predecessors.