Indications for Cetirizine
Cetirizine, better known as Zyrtec, is an antihistamine that can help control histamine-mediated pruritus by blocking the H-1 receptor, thereby inhibiting the inflammatory response triggered by the histamine. In layman's terms, Cetirizine helps to reduce any allergen-related (histamine-mediated) irritation (pruritus) by stopping the allergen pods (histamine) from reaching their docking stations (H-1 receptors), thereby preventing your body from sounding the allergy alarm (coughing, itching, and sneezing).
Since Cetirizine is a drug primarily intended for human use, and since there is no veterinarian-approved equivalent, Cetirizine for dogs and cats is considered an extra-label drug, or a drug being used outside its realm of approved use, even though it is commonly prescribed for pets by veterinarians.