The role of thymidine is to treat the diseases caused by the lack of immune cells and the defect of immune system, clinically it can be used to treat chronic hepatitis B, and lymphocyte deficiency, immunodeficiency, and asthmatic bronchitis, systemic lupus erythematosus, and chronic respiratory tract infection, such as chronic bronchitis, upper respiratory tract infection, mouth ulcer and so on, these can be treated by applying thymidine with medicine. But thymidine also has some side effects, the more common side effect is easy to have allergy, after all, human allergy is because of immune disorder, and thymidine can cause immune disorder, thymidine has bad effect on body, especially pregnant women and breastfeeding women, should be used with caution.