FINDING THE CORRECT BENADRYL DOSAGE FOR DOGS. The usual dose of Benadryl for dogs (adult) is 2 to 4 milligrams per kilogram of body weight, which corresponds to a dose of 0.9 to 1.8 miligrams per pound of body weight, according to the Merck Veterinary Manual.VERDOSE. Benadryl is a very safe drug for dogs, and it is unlikely that an owner would deliberately give a dog enough of the drug to be dangerous. Because dogs often eat things they should not, however, accidental overdose does happen. This almost always occurs when a dog gets into a box or bottle of Benadryl.