Mass. Gen. Laws ch. 201D § 5
An agent shall have the authority to make any and all health care decisions on the principal's behalf that the principal could make, including decisions about life-sustaining treatment, subject, however, to any express limitations in the health care proxy.
After consultation with health care providers, and after full consideration of acceptable medical alternatives regarding diagnosis, prognosis, treatments and their side effects, the agent shall make health care decisions: (i) in accordance with the agent's assessment of the principal's wishes, including the principal's religious and moral beliefs, or (ii) if the principal's wishes are unknown, in accordance with the agent's assessment of the principal's best interests.
Notwithstanding any general or special law to the contrary, the agent shall have the right to receive any and all medical information necessary to make informed decisions regarding the principal's health care, including any and all confidential medical information that the principal would be entitled to receive.
Health care decisions by an agent pursuant to a health care proxy on a principal's behalf shall have the same priority over decisions by any other person, including a person acting pursuant to a durable power of attorney as would decisions by the principal, when competent, except as otherwise provided in the health care proxy or by specific court order overriding the proxy.
A physician who is provided with a health care proxy shall arrange for the proxy or a copy thereof to be inserted in the principal's medical record.
A health care provider shall comply with health care decisions made by an agent under a health care proxy to the same extent as if such decisions have been made by the principal, subject to any limitations in the health care proxy, or in any specific court order.
Mass. Gen. Laws ch. 201D, § 5