What are the requirements in law for a valid consent to, or refusal of, medical treatment? Should it be possible for a doctor to override a competent patient’s refusal of life-sustaining medical treatment if the doctor considers it to be in the patients best interests?

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What are the requirements in law for a valid consent to, or refusal of, medical treatment? Should it be possible for a doctor to override a competent patient’s refusal of life-sustaining medical treatment if the doctor considers it to be in the patients best interests? Use case law to illustrate and support your answer.