The causes of fever again after the effect of antipyretics have passed are considered to be related to re-infection, autoimmune diseases, tumor diseases and so on. 1. Re-infection: some patients take antipyretic drugs after fever, and then have fever again after the effect of the drug has passed, which may be related to the factor of re-infection, which is mainly related to the low immunity during the period of disease. 2. Autoimmune disease: if the patient has lupus erythematosus or rheumatic immune disease, etc., it is likely to cause fever again after the effect of antipyretic drugs. 3. Tumor disease: If the patient has tumor disease, blood system disease or solid organ tumor, etc., it will also cause recurrent fever symptoms. In addition to the above factors, there are many other reasons for recurring fever, you can go to the hospital, cooperate with the doctor to improve the examination and treatment for the cause of the disease.