| Sumario: | Highlights: What are the main findings? Evidence for primarily ACT, ERITA, and IERITA interventions is promising but remains preliminary. The strongest current evidence concerns therapist-guided IERITA evaluated in a single randomized trial, while evidence for primarily ACT interventions and other ERITA formats remains preliminary. What are the implications of the main findings? Therapist-guided IERITA may warrant further evaluation as a promising adjunctive or stepped-care option for carefully selected adolescents; however, current evidence is insufficient to support its routine implementation or its use as a replacement for established treatments, particularly DBT-A for high-risk youth. Larger preregistered randomized trials with active comparators, longer follow-up, and mediation analyses are needed before any of these interventions can be considered established treatments for adolescent NSSI. Background/Objectives: Non-suicidal self-injury (NSSI) in adolescence is associated with emotion dysregulation, experiential avoidance, psychiatric morbidity, and later suicidal risk. Interventions based primarily on acceptance and commitment therapy (ACT) target psychological flexibility, acceptance, cognitive defusion, and values-based action, whereas emotion regulation individual therapy for adolescents (ERITA) and internet-delivered emotion regulation individual therapy for adolescents (IERITA) principally target emotion dysregulation while incorporating selected acceptance-related and ACT-consistent components, making these approaches theoretically relevant to adolescent NSSI. Methods: We conducted a systematic review and exploratory meta-analysis informed by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement. PubMed, Scopus, Web of Science, and EBSCO were searched on 10 April 2026. Eligible studies included adolescents or predominantly adolescent samples in which NSSI was the central clinical target or NSSI-specific outcomes were separately extractable and evaluated primarily ACT, ACT-informed, acceptance-based, ERITA, or IERITA interventions. Quantitative syntheses were restricted to controlled studies with extractable data. Embase, the Cochrane Central Register of Controlled Trials (CENTRAL), trial registries, and dedicated regional or gray literature sources were not searched; 15 reports remained unretrieved, so the evidence base may be incomplete. Results: Six full-text primary adolescent NSSI intervention studies met inclusion criteria: two controlled primarily ACT studies, one randomized ERITA feasibility trial, two uncontrolled ERITA feasibility/open studies, and one randomized IERITA trial. The strongest single-study evidence came from therapist-guided IERITA, which reduced masked assessor-rated NSSI frequency more than treatment as usual, with an incidence rate ratio of 0.34 (95% confidence interval [CI] 0.20 to 0.57). Two-study exploratory random-effects syntheses yielded estimates in the direction of benefit for continuous NSSI outcomes, Hedges' g = −0.45 (95% CI −0.85 to −0.05; k = 2), and process outcomes, Hedges' g = 1.25 (95% CI 0.83 to 1.68; k = 2). These estimates are hypothesis-generating and imprecise as estimates of a generalizable treatment effect because each synthesis contained only two clinically and methodologically heterogeneous studies. Although I2 was 0% in both syntheses, these estimates are highly uncertain with only two studies and should not be interpreted as evidence of true homogeneity. Using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach, certainty was moderate for the single masked clinician-rated IERITA outcome and very low for the pooled continuous NSSI and process-outcome evidence. Conclusions: Primarily ACT, ERITA, and IERITA interventions may be promising for adolescent NSSI, but the evidence remains preliminary and mostly of very low certainty. The pooled estimates are exploratory, hypothesis-generating, and imprecise and should not be interpreted as evidence of treatment efficacy. The strongest single-study evidence concerns therapist-guided IERITA and was rated as moderate in certainty, whereas the evidence for primarily ACT interventions and pooled process outcomes was very low in certainty and requires confirmation in larger preregistered randomized trials with standardized outcomes, longer follow-up, active comparators, and process measures.
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