Common side effects of Benadryl in canines include the presence of dry mouth, a loss or decrease in appetite with or without vomiting, diarrhea and a decrease in urine according to PedMD. Benadryl has been known to react with other prescription medications like amitraz, furazolidon and epinephrine. Special precautions must be taken when administering Benadryl, especially to pregnant canines and canines with heart diseases or high blood pressure.
Benadryl is an antihistamine that is commonly prescribed to treat allergy symptoms, a loss of appetite and increased serotonin production that is a result of Cushing's disease; however, it cannot treat the underlying cause of these problems. Dosages administered will be dependent on the type and severity of the symptoms, along with the size and type of canines that are being treated.