Tacrolimus immunosuppressant drug molecule. Computer model of a molecule of the immunosuppressant drug tacrolimus. Atoms are represented as spheres and are colour-coded; carbon (gold), hydrogen (white), oxygen (purple) and nitrogen (green). Tacrolimus works by inhibiting the enzyme calcineurin ,which is crucial for the multiplication of T cells. T cells are required for the activation of the immune system and inflammatory response. The drug is used to prevent rejection of transplanted organs and in topical form to treat eczema, an inflammatory skin disease.