The dosage of Benadryl for dogs is 1 mg/lb given every 8 hours (three times a day). For example, a dog weighing 40 lbs would need 40 mg of Benadryl per dose.enadryl for dogs suffering with allergies and other conditions. (redirects here from diphenhydramine). Donât use Benadryl for dogs without reading this first! The Benadryl dosage for dogs, safety precautions, side effects and other important information is included below.