Benadryl Dosage For Dogs. Note: The suggested dosage in this section is for Benadryl with the active ingredient diphenhydramine. The usual dosage for treating dogs is 1 mg/lb every 8 hours (three times daily) but can be doubled to 2mg/lb if needed.You shouldnât give any amount to your dog without checking with your vet first as the dosage can differ depending on several factors including your petâs medical history and breed.t the standard concentration every 5 mL of liquid contains 12.5 mg of diphenhydramine and can be given to dogs at a dosage of 0.4 mL/lb. For reference a US teaspoon measurement is 5 mL.