Cetirizine

**Cetirizine** is a **second-generation antihistamine** widely used in human medicine and increasingly utilized in veterinary practice for the adjunctive management of allergic and histamine-mediated pruritic (itchy) conditions. * **Non-sedating profile**: Unlike first-generation antihistamines (e.g., diphenhydramine, chlorpheniramine), cetirizine poorly penetrates the blood-brain barrier at standard doses, resulting in significantly less sedation. * **Active metabolite**: It is a piperazine-class derivative and the primary active human metabolite of hydroxyzine. * **Clinical utility**: Primarily used for atopic dermatitis in dogs and cats, adjunctive treatment of chronic rhinosinusitis in cats, and occasionally for allergic conditions in horses. While antihistamines are generally less effective as monotherapy for severe allergic flares in pets compared to humans, they are valuable as part of a multimodal, steroid-sparing approach.

กลไกการออกฤทธิ์: Cetirizine acts as a highly selective, competitive antagonist at peripheral **H1 receptors**. * **Allergic cascade**: Allergen exposure → Mast cell/basophil degranulation → **Histamine** release → Binds to **H1 receptors** on sensory nerve endings and blood vessels → Pruritus, vasodilation, and edema. * Cetirizine competitively blocks this binding, preventing histamine-mediated effects. * It also appears to decrease histamine release from basophils in some species, though experimental studies in cats showed it does not reduce eosinophilic airway inflammation. * It lacks significant anticholinergic or anti-serotonergic activity, and tolerance to its antihistaminic effects is not thought to occur.

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