Tacrofast 1mg Capsules (containing the active ingredient Tacrolimus) are primarily used to prevent your body from rejecting a newly transplanted organ, such as a kidney, liver, heart, or lung. Primary Medical Use Organ Transplant Rejection Prevention: When you receive a new organ, your immune system may view it as a foreign “invader“ and attack it. Tacrolimus is an immunosuppressant that weakens this immune response, helping your body accept and maintain the health of the new organ. It is typically prescribed in combination with other anti-rejection medications.