A relapse the day after an allergy shot may be due to not detaching from the allergen, allergy to the drug ingredients, insufficient drug dosage or course of treatment, and so on. 1. Not detached from the allergen: Since allergy medication can only alleviate the symptoms, if the allergy shot comes into contact with the allergen again, it may lead to a relapse the next day after the allergy shot. 2. Allergy to drug components: If the patient is allergic to a component of the drug, the allergy may also recur the next day after the injection. 3. Insufficient drug dosage or course of treatment: If the patient’s condition is severe, or the doctor uses insufficient drug dosage or course of treatment, the allergy may not be completely cured, which may also cause the allergy to recur the next day after the injection. There are many reasons for the allergy to recur the next day after the injection, it is recommended that the patient go to the hospital to clarify the allergens, and to avoid re-exposure, under the guidance of the doctor to treat the symptoms.