Generally, a patient may only be assessed and treated with his or her consent. It is difficult to interpret the law regarding doctors' powers and duties to assess and treat in cases where a patient refuses to accept medical advice but it is unclear whether the patient has decision-making capacity (DMC) or whether they are mentally ill. An examination of legal principles in this area suggests that there is a limited justification for doctors to detain a person for the purpose of assessment if they have good reason to suspect the person may either lack DMC or be mentally ill and if failure to detain the person is likely to lead to serious harm. This limited justification to detain would only apply for as long as the uncertainty about DMC or mental illness prevailed. Doctors have a duty to provide appropriate advice to patients who want to leave their care in circumstances where medical assessment or treatment may be required. Whether there is also a duty to detain someone in this situation is less clear, but we provide guidance to health care practitioners faced with this situation.