Ruxolitinib phosphate is primarily used in the treatment of myelofibrosis, a rare bone marrow disorder that disrupts the body's normal production of blood cells. It works by inhibiting the activity of certain enzymes that contribute to the overproduction of abnormal blood cells, helping to reduce symptoms such as an enlarged spleen, night sweats, and weight loss. Additionally, it is prescribed for polycythemia vera, a condition where the body produces too many red blood cells, leading to thickening of the blood and increased risk of complications like blood clots. In some cases, it is also used to manage steroid-refractory acute graft-versus-host disease (GVHD) in patients who have undergone stem cell transplants. The drug helps modulate the immune response, reducing inflammation and tissue damage associated with GVHD. Its targeted mechanism of action makes it a valuable option for these specific hematologic conditions.