However, according to vetpets.net (Vet Pets Animal Hospital in Cordova, TN), Diphenhydramine (Benadryl®) can be used to treat allergic reactions in dogs and cats. The primary side effect is drowsiness, although some cats and dogs, like some humans, may have varying side effects, from mild to severe. The dosage vetpets recommends is 1-2 mg per pound in weight every 8 hours.
Overuse will cause a pet to develop a tolerance to the medication and, at that point, it will become ineffective. Don't overuse!