Picking out an antihistamine for dogs can be a tiresome trial and error procedure. Yet, it is indispensable for your vet to be reassured the most effective antihistamine is found. Allergies require time to develop, but most dogs that develop an allergy do so between the ages of one and three years old. Of course other dogs may get an allergy much later, at perhaps six years old. In fact you never know when a dog may come down with an allergy. An itching dog is never a handsome sight and you must invariably be on the lookout for allergy trouble.