Benadryl®, also known by its generic name, diphenhydramine, is one of the few over-the-counter drugs designed for people that veterinarians may have pet parents administer at home.
You might be looking at using Benadryl® for dogs to keep your pup calm, or maybe your dog was stung by an insect and is having a mild allergic reaction. But while Benadryl® is generally well tolerated by dogs and has a wide safety margin, it’s not necessarily the answer to your dog’s issue.
For some dogs, giving Benadryl® to calm them may have the opposite effect and make them more anxious. In addition, Benadryl® should not be given to animals with certain health conditions or pups taking certain medications.
https://www.petmd.com/dog/care/can-i-give-my-dog-benadryl-and-if-so-how-much