The dosage is 1-2mg per pound of the dog's body weight up to 3 times per day. So a 20lb dog can have between 20-40mg 2 or 3 times daily.
I would definitely speak to your vet before treating him, just in case there are any health issues, or other medications with your dog, where Benadryl may not be advised. You could maybe speak briefly to the vet on the telephone. It won't cost anything.
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