Beginning on Day -2, you will receive tacrolimus by vein over 24 hours every day until you are able to take it by mouth. Tacrolimus is designed to weaken the immune system and lower the risk of graft-versus-host-disease (GVHD - a reaction of the donor's immune cells against your body). After you are able to take tacrolimus by mouth, you will take it every day for about 6 months, or until the doctor thinks it is safe to stop.