Yes, you can give your dog Zyrtec. In fact, Zyrtec is one of the safest over-the-counter medications for dogs. It is an excellent first-aid choice and has wide safety margins – it is not likely to cause any issues even if given in large doses.
Zyrtec is the brand name for the antihistamine cetirizine hydrochloride. Cetirizine is a sedative/antihistamine that works by blocking histamine activity in the body. Histamine plays a role in allergic reactions, such as those that occur afterratchy hair allergies or during hay fever season.