Used primarily as an immunosuppressant to prevent organ rejection after transplantation, especially in liver, kidney, and heart transplants. It works by inhibiting T-cell activation, reducing the body's immune response. Also used in the treatment of autoimmune diseases such as atopic dermatitis and certain cases of lupus nephritis. Available in oral and intravenous forms for systemic use, and as topical ointment for skin conditions. Commonly prescribed under medical supervision due to narrow therapeutic index and potential side effects.