Borderline Personality Disorder (BPD) involves intense emotional ups and downs, unstable relationships, impulsivity, and a poor sense of self. These challenges often lead to addiction, self-harm, and frequent use of healthcare services. While certain treatments already in practice - such as dialectical behavior therapy - can help, they don't work for everyone or address all aspects of the disorder. This study plans to explore internal family systems (IFS) therapy, a method that views the mind as made up of different "parts" - each with its own thoughts and feelings. IFS helps people build understanding and compassion toward these parts and connect with a calmer, more centered "Self." This may be especially useful for those with BPD, who often feel fragmented and extremely critical of themselves and others. This will be the first study to examine internal family systems therapy for BPD. The participants (15 in total) will receive up to 50 individual sessions over 15 months. Changes in symptoms and overall mental health will be measured at four points during the study. People with lived experience of BPD will help shape the research to ensure it is relevant, respectful, and useful for others facing similar challenges.

Brief background and rationale: Borderline personality disorder (BPD) is a chronic and disabling psychiatric condition marked by pervasive affective instability, identity disturbance, interpersonal dysfunction, and impulsivity. It affects approximately 1-2% of the general population and up to 20% of psychiatric populations, contributing substantially to healthcare utilization, suicide risk, and long-term disability. Existing evidence-based psychotherapies for this condition, such as dialectical behavior therapy, mentalization-based therapy, or schema therapy show significant benefits but these are modest and these interventions are quite resource-intensive. There remains a need for additional, theoretically grounded treatments that target core pathomechanisms of BPD-particularly emotion dysregulation, shame, and self-fragmentation.

Objectives: The primary objective of this pilot study is to assess preliminary effectiveness of internal family systems (IFS) therapy in reducing BPD symptom severity from baseline to post-treatment. Secondary objectives are to evaluate changes in emotion dysregulation, shame, self-compassion, interpersonal functioning, depression, anxiety, addictive behaviors and overall well-being; as well as to assess feasibility (recruitment, retention, adherence), acceptability, and safety. It is hypothesized that participants will demonstrate at least small-to-moderate improvements in BPD symptom severity, with congruent changes across secondary outcomes. Although clinical interest in IFS has grown exponentially in the past decades, empirical evaluation remains limited, and no studies to date have specifically examined its use in any personality disorder. Given the strong theoretical fit between IFS principles and BPD phenomenology, a structured pilot investigation is warranted to establish feasibility, safety, and preliminary effectiveness prior to larger controlled trials.

Study design: The study will use a single-group, repeated-measures design with assessments at baseline, 4 months, 8 months, and post-intervention (12-15 months). A target sample of fifteen adults will be recruited. Inclusion criteria are age 18 or older, sufficient English proficiency for participation, and a formal BPD diagnosis confirmed by a referring clinician. Exclusion criteria include active psychosis or mania, cognitive impairment, inadequate English comprehension, or participation in another form of psychotherapy during the study period.

Intervention: Internal family systems therapy, developed by Richard Schwartz, is an integrative psychotherapy that conceptualizes the mind as a system of subpersonalities ('parts') organized around a compassionate, stable core Self. The approach promotes internal harmony through increasing awareness, acceptance, and cooperation among these parts and the Self. Participants will receive weekly, one-hour IFS therapy sessions in the private mental health care setting over approximately 12 to 15 months. Treatment will be delivered by two licensed mental health professionals trained in the IFS model. Participants will pay a significantly subsidized session fee to reduce financial burden of participation in the intervention.

Outcomes: The primary outcome is BPD symptom severity measured by the Borderline Symptom List-23 and its supplement. Secondary outcomes include validated measures of emotion dysregulation (DERS-18), shame and guilt (GSQ-8), self-compassion (SCS-SF), interpersonal functioning (EXIS.pers), depression (PHQ-9), anxiety (GAD-7), addictive behaviors (SSBA-G), and well-being (WHO-5). Feasibility and acceptability will be evaluated through attendance, retention, and brief structured questionnaires.

Data collection and analysis: Data will be collected electronically via REDCap, using unique participant IDs and secure storage protocols. Analyses will include paired t-tests for pre-post differences and linear mixed-effects models for repeated measures across all four time points. Effect sizes will be prioritized over probability values when interpreting the results considering relatively low anticipated statistical power.

Timeline: The study begins in early 2026 following ethics approval, with recruitment and intervention delivery extending through the end of 2027. Data analysis, manuscript preparation, and dissemination are planned for 2028.

Inclusion Criteria:

• Resident of Ontario, Canada;
• Sufficient fluency in English to provide informed consent, participate in therapy sessions, and complete self-reported assessments;
• Diagnosis of borderline personality disorder confirmed by a healthcare professional authorized to establish such a diagnosis in Ontario;
• Willingness and ability (including financial means) to participate in the 50-session, 12-15-month intervention and the study assessments;
• Capacity and willingness to provide written informed consent.

Exclusion Criteria:

• Current manic or psychotic symptoms regardless of diagnosis;
• Substance dependence severe enough to interfere with meaningful level of engagement in the study intervention.
• Cognitive impairment or neurological disorders that would preclude informed consent or meaningful participation in the study intervention.
• Concurrent participation in any another form of psychotherapy.

A public repository dataset will be created from the anonymized data of those participants only who provided explicit consent during the informed consent process to allow their data to be stored in a data repository for secondary analyses. This dataset will be uploaded to a controlled-access repository (University of Toronto Dataverse within the Borealis network).

The data set will be available upon study completion, permanently.