The law of mental health authorizes involuntary detention in a psychiatric facility, in the absence of wrongdoing, of competent persons suffering from mental disorder likely to result in harm. Some jurisdictions are said to justify detention exclusively on the basis of danger prevention alone, thus predicating any ensuing treatment on the detainee’s categorical dignitary right to refuse treatment even while detained. Other jurisdictions are said to justify detention also on the basis of the detainee’s need of treatment, thus rendering the authorization to detain as a concomitant authorization to apply compulsory treatment. This article argues that ‘dangerousness’ is conceptually insufficient to justify detention in a psychiatric facility. If danger prevention per se were the only purpose of detention, then a psychiatric facility could perhaps be viewed as a permissible, but certainly not as the necessary, locus of authorized detention. Two propositions follow from this observation. The first is that need of treatment, in addition to danger prevention, must be a criterion of mental health law detention. The article formulates this proposition through a detailed reading of the detention provision in the Ontario (widely regarded as a ‘dangerousness’ jurisdiction) Mental Health Act and relevant case law. The second proposition is that, as much as the right to refuse treatment, regular and ongoing access to treatment is, therefore, a necessary condition of the justifiability, if any, of mental health law detention. The article develops this proposition through analysis of recent Ontario case law on the constitutionality of mental health law detention. The upshot is that failure to offer treatment is as inconsistent with constitutionally recognized liberty interests as the imposition of treatment in the absence of consent. By way of conclusion, the article focuses on what might be called the turn to criminal law to elaborate and affirm, by analogy, the rights of mental health law detainees. The article suggests that, while helpful, the turn to criminal law, because it brings into relief the dimension of danger to others, may also obscure the specific conceptual core of mental health law addressing danger to self.