Supplementary Material for: The Effectiveness of 6 versus 12 Months of Dialectical Behavior Therapy for Borderline Personality Disorder: A Noninferiority Randomized Clinical Trial

<b><i>Introduction:</i></b> Evidence-based psychotherapies for borderline personality disorder (BPD) are lengthy, posing a barrier to their access. Brief psychotherapy may achieve comparable outcomes to long-term psychotherapy for BPD. Evidence is needed regarding the comparative effectiveness of short- versus long-term psychotherapy for BPD. <b><i>Objective:</i></b> The aim was to determine if 6 months of Dialectical Behavior Therapy (DBT) is noninferior to 12 months of DBT in terms of clinical effectiveness. <b><i>Methods:</i></b> This two-arm, single-blinded, randomized controlled noninferiority trial with suicidal or self-harming patients with BPD was conducted at two sites in Canada. Participants (<i>N</i> = 240, <i>M</i> (SD)_age = 28.27 (8.62), 79% females) were randomized to receive either 6 (DBT-6) or 12 months (DBT-12) of comprehensive DBT. Masked assessors obtained measures of clinical effectiveness at baseline and every 3 months, ending at month 24. DBT-6 and DBT-12 were outpatient treatments consisting of weekly individual therapy sessions, weekly DBT skills training group sessions, telephone consultation as needed, and weekly therapist consultation team meetings. <b><i>Results:</i></b> The noninferiority hypothesis was supported for the primary outcome, total self-harm (6 months: margin = −1.94, <i>M</i>_diff [95% CI] = 0.16 [−0.14, 0.46]; 12 months: margin = −1.47, <i>M</i>_diff [95% CI] = 0.04 [−0.17, 0.23]; 24 months: margin = −1.25, <i>M</i>_diff [95% CI] = 0.12 [−0.02, 0.36]). Results also supported noninferiority of DBT-6 for general psychopathology and coping skills at 24 months. Furthermore, DBT-6 participants showed more rapid reductions in BPD symptoms and general psychopathology. There were no between-group differences in dropout rates. <b><i>Conclusions:</i></b> The noninferiority of a briefer yet comprehensive treatment for BPD has potential to reduce barriers to treatment access.