Y. Kotera, C. Newby, A. Charles, B.-R. Ingall, Y. Uneno, F. Ng, A. J. Sutton, L. J. Gray, E. A. Smith, E. Watson, L. Davidson, A. Simpson, S. Gillard, B. Puschner, S. A. Kidd, C. Mahlke, R. Nixdorf, L. Brophy, C. Brasier, A. Ashmore, S. Pomberth, T. A. Furukawa, M. Slade
One-to-one peer support is widely used in mental health services, but the components associated with better outcomes remain unclear. We systematically reviewed randomised controlled trials and conducted additive component network meta-analyses to identify which components of one-to-one peer support worker interventions were associated with outcomes for adults using mental health services. CINAHL Ultimate, Embase, MEDLINE, PsycINFO, CENTRAL, ClinicalTrials.gov and ISRCTN were searched, supplemented by citation tracking, previous reviews and expert consultation. Interventions were coded for seven components: Training and development, Maintaining peer support worker wellbeing, Relationship-building, Social support, Emotional support, Practical support and Cultural adaptation. The review followed PRISMA-NMA reporting guidance and was registered with PROSPERO (CRD42022355291). Thirty-six trials randomised 6,645 participants across nine countries. Only quality of life and recovery yielded estimable component effects at one or more follow-up points. For quality of life, Practical support had a positive incremental estimate at 3 months (standardised mean difference 0.52, 95% confidence interval 0.17 to 0.87); no component showed clear evidence of benefit at 6 months; and at 12 months Social support had a positive estimate (1.57, 0.12 to 3.01), whereas Maintaining peer support worker wellbeing had a negative estimate (-1.66, -3.05 to -0.28). These estimates were not consistent across follow-up points. For recovery, Relationship-building had positive estimates at 6 months (0.90, 0.03 to 1.78) and 12 months (0.50, 0.29 to 0.72). Networks were sparse and often disconnected, and additivity could not be tested in disconnected networks. Current trials do not permit definitive prioritisation of peer-support components. Relationship-building was the most consistent candidate component, but all findings remain provisional. Future trials should prospectively specify, manipulate and measure component delivery.