Diphenhydramine, hydroxyzine, cyproheptadine and chlorpheniramine are antihistamines that are safe for dogs. Fexofenadine, trimeprazine and cetirizine are also safe for dogs, according to the Merck Veterinary Manual.
Antihistamines given to dogs treat symptoms of allergies. Antihistamines relieve allergy symptoms but not the underlying cause. Antihistamines such as diphendramine can also treat Cushing's disease and motion sickness in dogs. Antihistamine dosage varies according to weight of the animal, and owners should only give them to dogs under veterinary guidance, advises PetMD.
Antihistamines may cause drowsiness, diarrhea, hyperexcitability and vomiting in dogs, and overdose of antihistamine can kill a dog. Some first-generation antihistamines, such as cyproheptadine, may cause increased appetite in dogs, reports the Merck Veterinary Manual.