Webaccording to veterinarians, the safe dosage of benadryl for dogs is 1mg per pound of body weight every 8 hours. Webthe lethal dose (ld 50) for diphenhydramine in dogs ranges between 24 to 30 mg/kg bw by iv administration, and death has been attributed to neuromotor excitement and. Webthe lethal dose of benadryl for dogs can vary depending on the size and weight of the dog.