loratadine

Loratadine competitively antagonizes peripheral H1 histamine receptors, blocking histamine-mediated effects such as vasodilation, increased vascular permeability, and pruritus. As a second-generation agent, it is more selective for peripheral H1 receptors and has low lipophilicity, limiting blood-brain barrier crossing and thus causing less sedation and cognitive impairment compared to first-generation antihistamines like diphenhydramine or chlorpheniramine. It also has minimal anticholinergic, antiadrenergic, and antiserotonergic activity, making it safer in elderly patients and those with conditions like BPH or glaucoma where anticholinergic effects are problematic. Clinically, loratadine is used for allergic rhinitis, allergic conjunctivitis, and chronic idiopathic urticaria. It is metabolized hepatically via CYP3A4 and CYP2D6 to desloratadine, its active metabolite (also marketed separately). Onset of action is within 1-3 hours, with once-daily dosing due to a long half-life (~8 hours, active metabolite ~27 hours). Unlike some second-generation antihistamines (e.g., terfenadine, now withdrawn), loratadine does not significantly prolong QT interval, making it relatively cardiac-safe. Key board-relevant comparison: first-generation antihistamines (sedating, anticholinergic, used for motion sickness/sleep aid) vs. second-generation (loratadine, fexofenadine, cetirizine - non-sedating, preferred for chronic allergy management).