Diphenhydramine is an antihistamine. It is used for the treatment of allergies and atopy, primarily to stop itching. (Atopy is an allergy to something that is inhaled, such as pollen or house dust; also called 'inhalant allergy.') It may also be used to control vomiting and motion sickness, as a mild sedative, and to treat muscle tremors due to certain toxins or medications. Contact your veterinarian if your pet experiences sleepiness, agitation, a rapid heart beat, depressed respiration, an inability to urinate, vomiting, diarrhea, or loss of appetite while being treated with diphenhydramine.