Purpose The evidence base for peer support work in mental health is established, yet implementation remains a challenge. The aim of this systematic review was to identify influences which facilitate or are barriers to implementation of mental health peer support work. Methods Data sources comprised online databases (n = 11), journal table of contents (n = 2), conference proceedings (n = 18), peer support websites (n = 2), expert consultation (n = 38) and forward and backward citation tracking. Publications were included if they reported on implementation facilitators or barriers for formal face-to-face peer support work with adults with a mental health problem, and were available in English, French, German, Hebrew, Luganda, Spanish or Swahili. Data were analysed using narrative synthesis. A six-site international survey [Germany (2 sites), India, Israel, Tanzania, Uganda] using a measure based on the strongest influences was conducted. The review protocol was pre-registered (Prospero: CRD42018094838). Results The search strategy identified 5813 publications, of which 53 were included. Fourteen implementation influences were identified, notably organisational culture (reported by 53% of papers), training (42%) and role definition (40%). Ratings on a measure using these influences demonstrated preliminary evidence for the convergent and discriminant validity of the identified influences. Conclusion The identified influences provide a guide to implementation of peer support. For services developing a peer support service, organisational culture including role support (training, role clarity, resourcing and access to a peer network) and staff attitudes need to be considered. The identified influences provide a theory base to prepare research sites for implementing peer support worker interventions.

The findings suggest that one-to-one peer support delivered by trained peer supporters can improve self-efficacy of patients with severe mental disorders over a one-year period. One-to-one peer support may be regarded as an effective intervention. Future research should explore the impact of improved self-efficacy on clinical and social outcomes.

Various types of peer support merit further evaluation. Assessing the impact of peer support on service users, peer providers and organizations require complex intervention studies, using mixed methods designs with qualitative exploration of underlying processes and experiences to complement high-quality controlled trials.