Tacrolimus is a powerful immunosuppressant drug, primarily used to prevent organ rejection (kidney, liver, heart, lung) after a transplant by calming the immune system, and also used topically (ointment) for severe eczema (atopic dermatitis). It works by inhibiting T-cell activation, reducing the body’s attack on new organs or inflamed skin, but carries risks like infection, kidney issues, high blood pressure, and diabetes, requiring careful monitoring. Common brand names include Prograf, Astagraf XL, and Protopic.