In psychiatry, practitioners are encouraged to adopt a patient‐centred approach that emphasises shared decision‐making. In this article, we investigate how clients with severe mental illnesses (e.g. schizophrenia) advocate for their treatment preferences in psychiatric consultations. The study uses Conversation Analysis to examine audio‐recorded medication check appointments in a comprehensive treatment programme known as assertive community treatment (ACT). The analysis shows that clients solicit medication changes at activity boundaries and design them in one of the following ways: reporting a physical problem; reporting a medication problem; explicitly requesting a medication change; and demanding a change. These formats put pressure on the psychiatrist to respond by either offering a solution to the client’s problem or by accepting or rejecting the client’s request. Through a detailed analysis of clients’ communicative behaviours, we show that, in soliciting a medication change, clients ordinarily respect boundaries of medical authority and present themselves as ‘good’ patients who are reliable witnesses of their own experiences. Overall, the paper advances our understanding of patient advocacy in psychiatry and mental health interactions more generally.