Treatment modifiers of interpersonal functioning in psychotherapy for people with borderline personality disorder: Systematic review with meta-analyses of individual participant data

BACKGROUND: Borderline personality disorder (BPD) is often accompanied by interpersonal dysfunction. Psychotherapy can improve interpersonal functioning, but individual characteristics may moderate outcomes. This systematic review used individual participant data meta-analysis (IPD-MA) to examine such moderators. METHOD: A literature search up to 26 November 2025 across 10 databases (including PubMed, Medline, Embase, PsychINFO, CINAHL, Web of Science, and Cochrane CENTRAL) identified randomised clinical trials (RCTs) investigating the effects of psychotherapy on interpersonal functioning in individuals with BPD compared to treatment as usual (TAU) or clinical management control interventions (CM). Authors of included trials were contacted to retrieve IPD. IPD-MAs employed a one-stage random-effects approach to estimate treatment effects on interpersonal functioning and potential moderators in bivariate linear mixed-effects models. The study was registered with PROSPERO (CRD42021210688). RESULTS: Out of 23,735 identified records, 32 RCTs (2762 participants) met inclusion criteria. Individual participant data (IPD) were available for 17 trials (1431 participants). All trials were rated as having either high risk of bias or some concerns. Missing data were common, with 321 out of 1431 participants (23%) lost to follow-up. Meta-analyses of both aggregate data and IPD yielded comparable effect estimates, though statistical significance differed (IPD-MA: β = -0.21, CI: -0.45 to -0.02, SE = 0.12, p = 0.0778; 17 trials, 1071 participants). In unadjusted analyses, the presence of co-occurring anxiety disorder(s) (β = -0.40, 95% CI: -0.73 to -0.08) and a higher number of co-occurring disorders (β = -0.08, 95% CI: -0.15 to -0.01) were associated with larger treatment effects (not significant after alpha correction). CONCLUSION: Psychotherapy appears to be effective for individuals with BPD. Although moderator effects did not remain statistically significant after alpha correction, unadjusted analyses suggested larger treatment effects in individuals with co-occurring anxiety and greater clinical complexity. Importantly, these findings indicate that such comorbidities may not be a contraindication for psychotherapy for BPD.