Fludarabine phosphate is primarily used in the treatment of certain types of cancer, particularly chronic lymphocytic leukemia (CLL). It is a chemotherapy drug that works by interfering with the growth and spread of cancer cells in the body. This medication is often administered when other treatments have not been effective. It is also used in conditioning regimens prior to hematopoietic stem cell transplantation, helping to prepare the body for the transplant by suppressing the immune system. Fludarabine phosphate is typically given intravenously and requires careful monitoring by healthcare professionals to manage potential side effects, which can include lowered blood cell counts and increased risk of infections.