This paper investigates legal and moral justifications of coerced treatment for psychiatric patients who are detained on the grounds that they may harm others. While the general issues concerning compulsory treatment and detention have been widely canvassed, it has seldom, if ever, been noticed that the moral reasons that we may have to detain a person who appears to be dangerous to others are different from the moral reasons we may have to treat him or her. For example, it has not been noticed that compulsory detention and compulsory treatment are supported by two different moral principles, namely the Principle of Harm and the Principle of Beneficence, and, therefore, that the arguments which support compulsory detention do not also support compulsory treatment. The conceptual confusion between legitimacy of compulsory detention and legitimacy of compulsory treatment is exacerbated by the ambiguous wording utilised in S 3 of the UK Mental Health Act, which implies that treatment may be necessary for the protection of others. Failure to pay attention to these distinctions has led to tragic consequences, in terms of violations of individual autonomy and in terms of public safety.