The evidence for the optimal duration of psychotherapy for borderline personality disorder (BPD) is scarce. Two previous trials have compared different durations of psychotherapy. The first compared 6 months versus 12 months of dialectical behaviour therapy for BPD (the FASTER trial). The second compared 5 months versus 14 months of mentalisation-based therapy for BPD (the MBT-RCT trial). The primary objective of the present study will be to provide an individual patient data pooled analysis of two randomised clinical trials by combining the two short-term groups and the two long-term groups from the FASTER and MBT-RCT trials, thereby providing greater statistical power than the individual trials. Accordingly, we will evaluate the overall evidence on the effects of short-term versus long-term psychotherapy for BPD and investigate whether certain subgroups might benefit from short-term versus long-term psychotherapy.

An individual patient data pooled analysis of the FASTER trial and the MBT-RCT trial will be conducted. The primary outcome will be a composite of the proportion of participants with a suicide, a suicide attempt or a psychiatric hospitalisation. The secondary outcome will be the proportion of participants with self-harm. Exploratory outcomes will be BPD symptoms, symptom distress, level of functioning and quality of life. We will primarily assess outcomes at 15 months after randomisation for the FASTER trial and at 16 months after randomisation for the MBT-RCT trial. Predefined subgroups based on the design variables in the original trials will be tested for interaction with the intervention as follows: trial, sex (male compared with female), age (below or at 30 years compared with above 30 years) and baseline level of functioning (Global Assessment of Functioning baseline score at 0–49 compared with 50–100).

The statistical analyses will be performed on anonymised trial data that have already been approved by the respective ethical committees that originally assessed the included trials. The final analysis will be published in a peer-reviewed scientific journal and the results will be presented at national seminars and international conferences.

CRD42024612840.