petMD reports that Benadryl is often prescribed to treat allergies in dogs. Walker Valley Veterinary Hospital says that dogs can be given 1 milligram of Benadryl per pound of the dog's weight twice a day to control allergy symptoms.
Benadryl is an antihistamine, says petMD. Benadryl is a brand name for a chemical called diphenhydramine. It acts to block the binding of histamine to H-1 receptors. During an allergic reaction, histamine is released and causes blood vessels to dilate and smooth muscles to contract. Benadryl blocks histamine from binding to the H-1 receptors and therefore relieves itching, swelling, inflammation and difficulty in breathing.