The causes of phlebitis include intravenous cannulation, input of irritating drugs and varicose veins, etc. The details are analyzed as follows: 1. Intravenous catheterization: long-term infusion and intravenous catheterization may damage and tear off the vein to cause microbial infection, leading to venous infectious inflammation. 2. Input of irritating drugs: for example, chemotherapy drugs and hypertonic drugs may lead to aseptic necrosis of blood vessels, which may cause inflammation, and redness, swelling, heat, pain, and streak-like manifestations in the local area. Generally, it needs to be relieved by cold compresses or taking medicines to improve the function of veins, such as Mazolin tablets, as prescribed by the doctor. 3. Varicose veins: In varicose veins, the blood stagnating in the veins may produce metabolic wastes, cytokines, etc. which stimulate the lining and cause thrombophlebitis. There are many causes of phlebitis, which need to be clarified before targeted treatment.