Tacrolimus, also known as fujimycin or FK506, is an immunosuppressant drug that is used after organ transplant procedures to prevent organ rejection. The drug acts by obstructing the activation of serine-threonine phosphatase, calcineurin, in T lymphocytes. This in turn hinders activation of T lymphocyte and halts the production of cytotoxic lymphocytes that otherwise nay lead to organ rejection. Tacrolimus is used in kidney and liver transplant. Organ transplanted patients often rely on immunosuppressant therapy during their lifetime. Tacrolimus is used for the treatment of heart transplant rejection, liver transplant rejection, kidney transplant rejection, and lung transplant rejection. The drug can be administered in various dosage forms such as tablets, capsules, and injections.