A dog becomes sleepy within 60 minutes from administration of a properly sized dose of Benadryl, according to AZVets.com. Peak levels of the active ingredient diphenhydramine are found in blood circulation within one to five hours, as noted in The Merck Veterinary Manual.
Proper dosage of Benadryl is 2 to 4 milligrams per kilogram of pet weight, as prescribed by The Merck Veterinary Manual. This agrees with the simple rule of 1 milligram per pound recommended by AZVets.com and DoseMyPet.com. As an example, a 30-pound dog should be administered 30 milligrams of Benadryl in a proper dosage.
Some over-the-counter products containing Benadryl also contain other medications that are toxic to canines, including acetaminophen or pseudoephedrine. AZVets.com recommends checking labels closely and obtaining the advice of a veterinarian before administering any drug to a dog.