Most OTC antihistamines can be used in dogs; however you should never give any medications to your dog without the advice of your veterinarian. Allergies can be frustrating to diagnose since they can have multiple causes (inhaled, food, contact). Schedule an appointment with your veterinarian. Discuss food elimination trials and allergy testing and if antihistamine use is indicated, he or she can give you a proper dose, The medication may provide some relief but your task is to try to determine what your dog is allergic to and decrease or eliminate it. Monitor her for side effects including lethargy and weakness, and if you see those signs, have her seen by a vet right away. Best of luck.