There are three lines of argument in defence of coercive treatment of patients with mental disorders: arguments regarding (1) societal interests to protect others, (2) the patients' own health interests, and (3) patient autonomy. In this paper, we analyse these arguments in relation to an idealized case, where a person with a mental disorder claims not to want medical treatment for religious reasons. We also discuss who should decide what in situations where patients with mental disorders deny treatment on seemingly rational grounds. We conclude that, in principle, coercive treatment cannot be defended for the sake of protecting others. While coercive actions can be acceptable in order to protect close family and others, medical treatment is not justified for such reasons but should be given only in the interest of patients. Coercive treatment may be required in order to promote the patient's health interests, but health interests have to waive if they go against the autonomous interests of the patient. We argue that non-autonomous patients can have reasons, rooted in their deeply-set values, to renounce compulsory institutional treatment, and that such reasons should be respected unless it can be assumed that their new predicaments have caused them to change their views.