According to veterinarian Dr. Kara McCarty, it is safe to give a puppy Benadryl. Puppies are dosed at 1 milligram of Benadryl per pound of body weight three times a day. A 10- to 12-pound puppy can receive half of a 25 milligram Benadryl tablet three times per day.
As PetMD points out, Benadryl, which is the brand name for the over-the-counter antihistamine diphenhydramine, is used to treat symptoms associated with an overproduction of histamine. The body produces histamine in response to allergies; associated symptoms include itching, inflammation and difficulty breathing. Benadryl is also used to treat motion sickness and hyperexcitability in dogs. Only give Benadryl to dogs and puppies when advised to do so by a veterinarian.