The experience-based knowledge of users is considered to provide vital input in shared decision making (SDM). However, mental health service users frequently express having negative experiences from meetings with providers, which are of an epistemic nature (e.g., being ignored or not regarded as credible). This study aimed to explore the barriers involved in legitimizing user knowledge in decision-making processes. Interview data from service users and providers were viewed from a theoretic framework of epistemic injustice. Abductive content analysis was conducted on data collected during a project to develop and implement SDM in mental health services. In describing obstacles to legitimize user knowledge, service users highlighted relational issues: being dependent, often dismissed and choosing to edit their testimonies. Service providers typically described workflow issues, users’ insufficient decision- making competence and users’ vulnerability to stress factors. The findings suggest that greater epistemic justice might be achieved by a SDM process in which the service user is engaged as a full partner in collaboration in various activities related to their care.