Cyclophosphamide (CTX) has a significant role in the treatment of connective tissue diseases such as lupus nephritis and systemic vasculitis, but care should be taken to monitor toxic side effects. The common acute side effects include nausea, vomiting, diarrhea, alopecia, low sodium and reversible myelotoxicity such as decreased blood leukocytes and decreased platelets, but recovery begins after 2 weeks. The common side effects of long-term application are: 1. malignant tumors: there is a correlation with the total dose of CTX, and the chance of tumor occurrence increases significantly above 30 grams, mostly malignant tumors of the urinary tract, bone marrow, and skin; 2. hemorrhagic cystitis: generally occurs after oral administration of CTX, and most of them do not drink a lot of water after taking the drug. Some people have been confirmed that the chance of bladder tumor after the occurrence of hemorrhagic cystitis increases significantly, so it is called pre-cancerous damage; 3, reproductive toxicity: after the application of CTX in women, especially after middle age, can lead to ovarian failure, manifested as amenorrhea, sterility, and in pregnant women, can also appear teratogenic; and in men, the formation of testicular atrophy, loss of libido, azoospermia or sperm weakness; 4, infection. Since CTX can lead to a decrease in the immunity of the body, it is easy to form various bacterial and viral infections, especially some opportunistic infections. To sum up, CTX has some serious toxic side effects, but we should not “choke on it” for this reason. The correct management of this contradiction depends on strict control of indications and close observation of various toxic side effects.