According to PetMD, Benadryl dosages can vary and a veterinarian should be consulted for the proper amount. Although Benadryl is available over-the-counter, do not give a dog any before taking it to the vet. Benadryl is given to dogs to treat allergies, motion sickness and hyperactivity.
Benadryl treats the symptoms of allergies, appetite loss and increased serotonin production, but it does not address the underlying cause of these problems. It works by counteracting histamines, thus preventing small blood vessels from widening and causing inflammation and itchiness. Blocking the histamines also prevents the smooth muscles from contracting and restricting airways. Unique to dogs, Benadryl also blocks the portion of the brain that controls the vomiting impulse, which is why it is prescribed to treat motion sickness.
Side effects of Benadryl include decreased urination, dry mouth, lethargy, diarrhea, loss of appetite and vomiting. It can have negative effects on dogs with heart disease, high blood pressure, bladder disorders and hyperthyroidism. Although it is not strictly prohibited to dose pregnant dogs with Benadryl, owners who choose to do so should exercise caution.
Signs that a dog may need Benadryl include itchy, red or scabbed skin, runny eyes and nose, chronic ear infections, skin infections, sneezing and constant paw chewing or licking.