An advance decision, also known as a "living will" or "advance directive," is a legal document that allows an individual to set out their wishes for medical treatment in the event that they are unable to make those decisions themselves. This type of document is particularly important for individuals who may be at risk of losing mental capacity due to illness or injury.

In the United Kingdom, an advance decision can be used to refuse specific medical treatments that the individual does not wish to receive. This includes treatments such as life-sustaining treatment, such as artificial feeding and hydration, or certain types of medical procedures. The document must be legally binding, and must comply with the requirements set out in the Mental Capacity Act 2005.

It is important to note that an advance decision is different from a lasting power of attorney for health and welfare, which allows an individual to appoint someone to make decisions on their behalf if they lose capacity. An advance decision is used to refuse treatments, whereas a lasting power of attorney for health and welfare is used to appoint someone to make decisions about treatments that the individual may wish to receive.