According to PetMD, Benadryl is sometimes used to treat dogs for conditions that include allergies, motion sickness and symptoms of Cushing's disease, which is caused by a benign pituitary gland tumor. Although it is an over-the-counter medication, Benadryl should not be given to a dog without instructions from a veterinarian.
PetMD explains how Benadryl, an antihistamine, works. In dogs with allergies, the drug counteracts the inflammation and itchiness caused by the chemical "histamine." Benadryl also has a unique effect on dogs: it prevents messages from being sent to the part of the brain that regulates vomiting. Therefore, it is helpful in treating canine motion sickness. Possible side effects include drowsiness, vomiting, diarrhea and appetite loss.