I've been told by my vet, you can give a dog a mg/lb of benadryl R.Jennings RN ***Improvement*** A 12 lb dog should receive 6-24 milligrams. Start low because it can cause ups â¦ et stomach. The dosage for Diphenhydramine in dogs is 0.5 to 2 mg per pound.The liquid comes in 12.5mg per 5ml.I tried to give some to my dog and it was a mess! Pills are much easier to hide in food. You can usually get 40 25mg pills for $1 at a store like the dollar general.tart low because it can cause ups â¦ et stomach. The dosage for Diphenhydramine in dogs is 0.5 to 2 mg per pound. The liquid comes in 12.5mg per 5ml. I tried to give some to my dog and it was a mess! Pills are much easier to hide in food.