Transfusion of platelets is important for the care of patients undergoing stem cell transplantation. Stem cell transplants are used to treat blood cancers and bone marrow disorders. This involves the use of high doses of chemotherapy and/or radiation to kill cancer cells; but this damages the marrow and blood system. Blood stem cells are transplanted by infusing into the recipient and blood counts recover over 2-3 weeks. Before bone marrow recovery, platelets are needed to support the patient. Platelets in these high risk patients may prevent bleeding while the blood counts recover. Tranexamic acid (TA) is an oral medication that can used to prevent bleeding. Research in other areas of medicine has shown that TA is safe and can be used to prevent bleeding. However, TA has not been studied in stem cell transplant patients. In this study, we will compare platelet transfusions with TA. We will do this by randomly assigning patients having a transplant to receive either platelets or TA. In this way, we will be able to accurately find out if there are any benefits or harms in either approach.