How much Benadryl can a dog have and how often?

The recommended dosage of Benadryl for dogs is 1 milligram for every 1 pound of the dog's weight, according to VetInfo. This can be administered every 8 hours on the advice of a veterinarian.

VetInfo explains that Benadryl is an antihistamine that can be administered to dogs to alleviate allergic reactions, itching, car sickness and irritation from insect bites. It also has mild sedative effects.

It is important to ensure that plain Benadryl is administered and not a formulation that includes another medication that may be harmful to the dog, VetInfo warns. Because the liquid formulation of Benadryl includes a high alcohol content, only Benadryl pills or capsules are appropriate for use in dogs.