Used primarily as an immunosuppressant to prevent organ rejection after transplantation, especially in liver, kidney, and heart transplant patients. 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 nephrotic syndrome. Available in oral and intravenous forms for systemic use, and as topical ointment for skin conditions. Its potent immunosuppressive effect requires careful blood level monitoring to balance efficacy and risk of side effects.