Yes, a competent adult can refuse treatment in a hospital, even if the refusal could result in death. Healthcare professionals are required to discuss the decision with you, answer your questions, and make sure you understand the risks before accepting your refusal. If you lack the capacity to make a decision, and have not made an advance decision (a "living will"), doctors will decide what is in your best interests, consulting with relatives where possible.
The British Medical Association's Core Ethic Guidance states that persons may refuse treatment -
'...except where compulsory treatment for the patient’s psychiatric disorder is authorised by mental health legislation. The situation is different for patients under the age of 18; for guidance on children and young people under 16, and guidance on 16 and 17-year-olds, see key resources. Doctors must respect a patient’s decision to refuse treatment, even if they do not agree with it or when it could lead to permanent injury or death. For example, a Jehovah’s Witness can refuse a blood transfusion even where this is essential for survival. Providing treatment without valid consent, and in the face of a competent refusal, would leave the doctor open to legal and professional sanctions. A refusal of treatment should not be interpreted as a refusal of all treatment or care. It is essential that steps are taken to keep the patient comfortable and that any symptoms or distress are appropriately managed. Patients are not required to justify their decision to refuse consent, but healthcare professionals should seek to ensure that patients base their decisions on accurate information and that they have corrected any misunderstandings. Patients also need to know if refusing treatment now will limit their future options.
Doctors must not put pressure on patients to decide in a particular way, but should allow them time to consider a decision with potentially serious consequences.'
More information can be found on the websites in Related Information.
