Since the mid-19th century doctors have exercised almost exclusive control in health professionals' decisions concerning compulsory assessment and treatment of the mentally ill. This control has the potential to compromise the legitimate professional practice of other mental health professionals. A new approach to mental health legislation has seen the involvement of a range of health professionals in legislated mental health roles, including the power of registered nurses to detain patients in hospital under Section 111 of the New Zealand Mental Health (Compulsory Assessment and Treatment) Act (1992). Under this Section a nurse who believes that a voluntary patient meets the legal criteria of the Act can independently detain the patient for a period of up to 6 hours, pending further assessment by a medical practitioner. However, anecdotal evidence and a clinical audit undertaken by the authors suggest some doctors 'prescribe' Section 111 at the time of admission. This practice instructs nurses to initiate Section 111 if particular voluntary patients choose to leave hospital. This study outlines practice issues resulting from 'prescribing' Section 111; provides a legal critique of medical practitioners' involvement in this practice; and makes recommendations for guidelines toward a more constructive use of Section 111.