Supplementary Material for: Short-term versus long-term mentalization-based therapy for borderline personality disorder (MBT-RCT): 24 months follow-up of a randomized clinical trial

Introduction Borderline personality disorder (BPD) is a severe and prevalent psychiatric disorder. Mentalization-based therapy (MBT) is an evidence-based intervention for BPD, which is often delivered as a long-term psychotherapy program for BPD. We previously published a randomized clinical trial assessing short-term versus long-term MBT for BPD 16 months after randomization as the primary follow-up time-point. Objectives To assess the long-term (24 months) results of short-term versus long-term MBT for outpatients with BPD. Methods Adult outpatients (≥18 years) with subthreshold or diagnosed BPD were randomly assigned (1:1) to short-term MBT (5 months) or long-term MBT (14 months). The primary outcome was BPD symptoms assessed with the Zanarini Rating Scale for Borderline Personality Disorder. Secondary outcomes were level of functioning (assessed using the Work and Social Adjustment Scale), quality of life (assessed using Short Form Health Survey, SF-36), global functioning (assessed using the Global Assessment of Functionins scale, GAF), and severe self-harm. All outcomes were assessed at 24 months after randomization. Results Between October 4, 2018, and December 3, 2020, we randomly assigned 166 participants to short-term MBT (n=84) or long-term MBT (n=82). After 24 months, regression analyses showed no evidence of a difference when assessing the primary outcome, BPD symptoms (ZAN-BPD MD -0.56; 95% CI -2.67 to 1.54; p = 0.598), level of functioning (WSAS MD -1.42% CI -5.04 to 2.21; p = 0.440), global functioning (GAF MD 2.51 95% CI: -1.65 to 6.67; p = 0.234), or severe self-harm (RR 1.38; 95% CI 0.88 to 2.21; p = 0.149). Regression analyses showed evidence of a beneficial effect of long-term MBT when assessing