If an adult lacks the capacity to give consent, a decision on whether to go ahead with the treatment will need to be made by the health professionals treating them. In order to make a decision, the person's "best interests" must be considered.
There are many important elements involved in trying to determine what a person's best interests are, including:
considering whether it is safe to wait until the person can give consent, if it is likely they could regain capacity at a later stage
involving the person in the decision as much as possible
trying to identify any issues the person would take into account if they were making the decision themselves, including religious or moral beliefs; these would be based on views the person expressed previously, as well as any insight close relatives or friends can offer
If a person is felt to lack capacity, and there is no one suitable to help make decisions about medical treatment, such as family members or friends, an independent mental capacity advocate (IMCA) must be consulted.