BACKGROUND: People seeking care for substance use (PSCSU) experience deep social and health inequities. Harm reduction can be a moral imperative to approach these persons. The purpose of this study was to explore relationships among users, health care providers, relatives, and society regarding harm reduction in mental health care, using a trust approach rooted in feminist ethics. METHODS: A qualitative study was conducted in a mental health service for PSCSU, and included fifteen participants who were health care providers, users, and their relatives. Individual in-depth and group interviews, participant observation, and a review of patients' records and service reports were conducted. RESULTS: Three nested levels of (dis)trust were identified: (dis)trust in the treatment, (dis)trust in the user, and self-(dis)trust of the user, revealing the interconnections among different layers of trust. (Dis)trust at each level can amplify or decrease the potential for a positive therapeutic response in users, their relatives' support, and how professionals act and build innovations in care. Distrust was more abundant than trust in participants' reports, revealing the fragility of trust and the focus on abstinence within this setting. CONCLUSION: The mismatch between wants and needs of users and the expectations and requirements of a society and mental health care system based on a logic of "fixing" has contributed to distrust and stigma. Therefore, we recommend policies that increase the investment in harm reduction education and practice that target service providers, PSCSU, and society to change the context of distrust identified.