BackgroundOnline forums are widely used for mental health peer support. However, evidence of their safety and effectiveness is mixed. Further research focused on articulating the contexts in which positive and negative impacts emerge from forum use is required to inform innovations in implementation.ObjectiveTo develop a realist programme theory to explain the impacts of online mental health peer support forums on users.MethodsRealist synthesis of literature published 2019-2023 and 18 stakeholder interviews with forum staff. ResultsSynthesis of 102 evidence sources and 18 interviews produced an overarching programme theory comprising 22 context-mechanism-outcome configurations. Findings indicate that users’ perceptions of psychological safety and the personal relevance of forum content are foundational to ongoing engagement. Safe and active forums that provide convenient access to information and advice can lead to improvements in mental health self-efficacy. Within the context of welcoming and non-judgmental communities, users may benefit from the opportunity to explore personal difficulties with peers, experience reduced isolation and normalization of mental health experiences, and engage in mutual encouragement. The programme theory highlights the vital role of moderators in creating facilitative online spaces, stimulating community engagement, and limiting access to distressing content. A key challenge for organizations that host mental health forums lies in balancing forum openness and anonymity with the need to enforce rules, such as restrictions on what users can discuss, in order to promote community safety.ConclusionThis is the first realist synthesis of online mental health peer support forums. The novel programme theory highlights how successful implementation depends on establishing protocols for enhancing safety and strategies for maintaining user engagement to promote forum sustainability.<br/
