Side effects from allergy immunotherapy drops are mostly mild. Tingling or itching in the mouth is the most common side effect but generally resolves in 1 to 2 weeks. In rare cases, mild nausea or other infrequent, short-lived abdominal discomfort can occur. As long as these symptoms are minor and resolve quickly, you may continue with your drops as prescribed. In very rare cases, more significant side effects such as breathing difficulties or swelling may be experienced. If any reaction is worrisome, use your epinephrine autoinjector and immediately dial 911 or proceed to the nearest emergency room.