Non-suicidal self-injury and subsequent suicide attempts and death: A protocol for a systematic review of temporally ordered evidence.

BACKGROUND: Non-suicidal self-injury (NSSI) is a prevalent and clinically significant behaviour, particularly among adolescents and young adults, and is strongly associated with suicidal behaviour. However, existing reviews have often combined heterogeneous outcomes, including suicidal ideation, plans, attempts, and death, limiting the identification of clinically meaningful pathways. OBJECTIVE: This protocol describes a systematic review that aims to synthesise evidence on the temporally ordered relationship between NSSI and subsequent suicide attempts or death, and to examine whether characteristics of NSSI are associated with increased risk or severity of later suicidal behaviour. METHODS: This protocol is reported in accordance with PRISMA-P 2015 and registered with the Open Science Framework (OSF; https://doi.org/10.17605/OSF.IO/EA4M6). A comprehensive search will be conducted in MEDLINE, Embase, PsycINFO, CINAHL, and Web of Science. Eligible studies will include prospective and retrospective observational studies demonstrating temporal ordering between NSSI and subsequent suicide attempts or death. Studies focusing solely on suicidal ideation will be excluded. Data extraction and risk-of-bias assessment will be conducted independently by two reviewers using Joanna Briggs Institute tools. Due to anticipated heterogeneity, findings will be synthesised using a structured narrative approach. CONCLUSION: This review will provide a focused synthesis of evidence on the relationship between NSSI and subsequent suicidal outcomes, addressing gaps related to temporality, outcome specificity, and severity. The findings are expected to inform clinical formulation, safety planning, and future research on suicide prevention by improving understanding of factors associated with subsequent suicidal behaviour.