The possible side effects of sublingual immunotherapy, or SLIT, include itching, irritation or swelling in the mouth, gastrointestinal upset and, in rare occurrences, anaphylaxis, states the American College of Allergy, Asthma and Immunology. Anaphylaxis is the most severe side effect that has been associated with sublingual immunotherapy.
Sublingual immunotherapy involves the dispensing of liquid drops or tablets containing allergen extracts under the tongue to increase the immune system's resistance to an allergen and to decrease its associated symptoms, explains American College of Allergy, Asthma and Immunology. This method of immunotherapy is relatively safe, but since it is typically administered at home and not at the office of an allergist, the potential for incorrect administration of the drops to the allergy sufferer can reduce the effectiveness of the therapy and increase the risk of adverse reaction to the treatment.
Additionally, sublingual immunotherapy requires long-term use of allergy drops in gradually increasing amounts to slowly build up a person's tolerance to an allergen or allergens, states the Australasian Society of Clinical Immunology and Allergy. Allergists recommend a period of treatment of three to five years to increase the likelihood that the allergy or allergies will not return.
Anaphylaxis is the most serious potential side effect of sublingual immunotherapy, says the American College of Allergy, Asthma and Immunology. Symptoms of anaphylaxis include hives, itching, flush or pale skin, airway constriction, swollen tongue, nausea, and dizziness, states Mayo Clinic. Anaphylaxis is a medical emergency.