You can give the antihistamine Benadryl to your dog, but you should always check with your vet first. Benadryl can be used to treat dogs with Hives, Swelling, Cough, Sneezing, Runny eyes or nose. The safest way to give Benadryl is to give it according to the weight of your dog. AKC says the dose for Benadryl is 2 to 4 milligrams per kilogram of body weight, two to three times per day. Other antihistamines can also be given to dogs, based on their weight. These include the below antihistamines: Cetirizine, Clemastine, Chlorpheniramine, Cyproheptadine, Fexofenadine, Hydroxyzine, Terfenadine, Trimeprazine. The safest dose is different for different antihistamines, so always check with your vet first. Cetirizine 1 milligram per kilogram of body weight or 10 to 20 milligrams per dog, once or twice per day. Chlorpheniramine Dogs under 20 kilograms 4 milligrams, three times per day, Dogs over 20 kilograms 8 milligrams, three times per day. Cyproheptadine 0.25 to 0.5 milligrams per kilogram of body weight, three times per day. Fexofenadine 2 to 5 milligrams per kilogram of body weight, once or twice per day. Hydroxyzine 0.5 to 2 milligrams per kilogram of body weight, three to four times per day.