Adverse reactions to allergenic extracts are extremely rare when administered orally. You should always observe your pet for 20-30 minutes after treatment. Signs and symptoms of possible side effects could include the following: facial swelling, redness, itching and tingling of the mouth, excessive drooling, sneezing and panting. Usually a simple reduction in volume given is all that is required to eliminate side effects. Your veterinarian may suggest giving liquid anti-histamine such as over-the-counter, generic cetirizine or diphenhydramine.