"Advance directive" is a general term that describes two types of legal documents:
Medical power of attorney
These documents allow you to instruct others about your future medical care wishes and appoint a person to make healthcare decisions if you are not able to speak for yourself. Each state regulates the use of advance directives differently.
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A living will is a type of advance directive in which you put in writing your wishes about medical treatment for the end of your life in the event you cannot communicate these wishes directly. Different states name this document differently: for example, it may be called a "directive to physicians," "health care declaration," or "medical directive." Regardless of what it is called, its purpose is to guide your family and doctors in deciding about the use of medical treatments when you are dying.
Your legal right to accept or refuse treatment is protected by the Constitution and case law. However, your state law may define when the living will goes into effect, and may limit the treatments to which the living will applies. You should read your state's suggested document carefully to ensure that it reflects your wishes. You can add further instructions or write your own living will to cover situations that the state suggested document might not address. Even if your state does not have a living will law, it is wise to put your wishes about the use of life-sustaining medical treatments in writing.
WebMD Medical Reference from the National Hospice and Palliative Care Organization