Cytoxan (sītŏk`sĭn), trade name for the drug cyclophosphamide, used to inhibit growth of tumors and rapidly proliferating cells. It is used in the treatment of leukemia, Hodgkin's disease, and lymphosarcoma and other solid tumors. A drug of great specificity, Cytoxan is activated in the body by the enzymes in certain types of tumors. Because it is also activated in such tissues as blood plasma and liver, it is a highly toxic substance. Nevertheless, it is a potent and useful chemotherapeutic agent.