Many rheumatic patients are often surprised when they go to the hospital, read the drug instructions, and find that they are on anti-cancer drugs such as methotrexate and cyclophosphamide. Why is this? Let’s go through the following contents to understand. First of all, let’s understand what is “rheumatism”? From the perspective of Western medicine, rheumatism is not a disease caused by “wind” and “humidity”, but an inflammatory disease caused by various factors (including genetics, injury, infection, etc.), such as systemic lupus erythematosus, rheumatoid arthritis, dermatomyositis/polymyositis, systemic vasculitis, etc. Systemic vasculitis, etc., all belong to the category of rheumatic diseases. They are all diseases caused by “autoimmunity”. Autoimmunity” is simply understood as an attack by immune cells on oneself. Under normal circumstances, the main function of the human immune system is to destroy foreign invaders, such as various bacteria and viruses. When the immune system is disrupted and “autoimmunity” occurs, it is like a rebellion in your own army, where your own people are attacking your own people. In this case, the immune cells are very active, and their “killing power” is very great, so the disease is very serious. Then, why can anti-cancer drugs kill the “betrayed” immune cells? We know that the more active cells proliferate, the more sensitive they are to anti-cancer drugs, while the less active cells proliferate, the less sensitive they are to anti-cancer drugs. In rheumatic disease patients, their own immune cells are very active, proliferate rapidly and produce a large number of auto-antibodies, which can damage various organs. Since anti-cancer drugs are very powerful to kill the proliferating cells, they can kill the immune cells against themselves and are effective for rheumatism treatment. However, it does not mean that anti-cancer drugs do not damage normal human body. In human body, blood cells, liver cells, gonadal cells, etc. are renewed faster, and anti-cancer drugs have an effect on them. Therefore, in clinical practice, we still need to pay attention to monitor blood routine, liver function, etc., and colleagues should pay attention to protect gonadal function.