Public health strategies for first-episode psychosis: a systematic review of interventions and modifiable influences.

Background. First-episode psychosis (FEP) refers to the first occurrence of psychotic symptoms severe enough to impair perception, cognition, and functioning and to warrant clinical attention. Although FEP is most common in late adolescence and early adulthood, it can occur across the life course. A consistent staging model describes its development through phases of vulnerability, clinical high-risk or prodromal symptoms, and first-episode onset. Despite advances in detection and early intervention, the evidence base for prevention remains fragmented. Most reviews have focused narrowly on clinical high-risk samples, overlooking the broader ecological, behavioural, and structural determinants of psychosis risk. At the same time, longitudinal research has identified multiple modifiable factors, such as trauma, substance use, discrimination, and social disadvantage, that operate across the life course but have not been synthesised systematically. These linked reviews will address these evidence gaps by examining preventive interventions and modifiable risk factors for FEP. Methods. Two systematic reviews will be conducted in accordance with the Cochrane Handbook and reported following PRISMA 2020. Review 1 will synthesise evidence for the effectiveness of interventions aimed at preventing FEP among individuals without prior psychosis, in four bibliographic databases. Review 2 will synthesise longitudinal evidence on modifiable behavioural, experiential, and structural risk factors for FEP onset, in four bibliographic databases. Two reviewers will independently screen, extract, and appraise studies using EndNote 21, Rayyan, and Excel. Risk of bias will be assessed using the most current Cochrane checklists in both reviews. Grading of Recommendations Assessment, Development and Evaluation (GRADE) will be applied to rate certainty of evidence in Review 1. Narrative synthesis and, where feasible, random-effects meta-analyses will be undertaken. Dissemination. Ethical approval will not be required. Findings will be disseminated through peer-reviewed publications, conference presentations, and stakeholder reports. Public involvement and engagement will be embedded throughout the review process, following NIHR guidance, to ensure that lived-experience perspectives inform interpretation and dissemination.