Psilocybin Therapy in TRD with Cluster B Personality Disorders: First Clinical Trial Findings

First Clinical Trial: Psilocybin-Assisted Psychotherapy in TRD with Cluster B Personality Disorders

The first clinical trial to examine psilocybin-assisted psychotherapy (PAP) in adults with both treatment-resistant depression (TRD) and cluster B personality disorders (PDs) has reported initial safety and feasibility outcomes. This secondary analysis, published on September 16, 2026, describes a subgroup of ten participants (nine with cluster B PDs, one with cluster C) from a larger registered trial (NCT05029466). The study addresses a critical gap: individuals with both TRD and cluster B PDs—such as borderline, narcissistic, histrionic, or antisocial personality disorders—often respond poorly to standard antidepressants and are frequently excluded from clinical trials.

Mechanism and Clinical Context: Why This Population Matters

Cluster B PDs are characterized by emotional dysregulation, impulsivity, and interpersonal difficulties, which complicate both the course of depression and its treatment. Conventional pharmacotherapies show limited efficacy in this group, partly due to high comorbidity and complex psychosocial factors. Psilocybin, a serotonergic psychedelic, is hypothesized to promote neuroplasticity and facilitate psychological flexibility, potentially addressing entrenched cognitive and emotional patterns resistant to standard interventions. In this trial, participants received PAP in a structured therapeutic setting, with depression severity measured using the Montgomery-Åsberg Depression Rating Scale (MADRS) and suicidal ideation tracked as a key safety endpoint.

Key Findings: Safety, Tolerability, and Heterogeneity of Outcomes

The trial found no serious adverse events among participants, supporting the preliminary safety and tolerability of PAP in this high-risk population. Mean MADRS scores decreased from 30.4 pre-treatment to 22.5 at day 1, 24.9 at week 2, and 22.3 at month 6. Suicidal ideation, as measured by the MADRS SI item, declined from 3.2 to 1.6 at day 1 and further to 1.2 at month 6. However, outcomes varied substantially between individuals, and three of the ten participants dropped out, underscoring challenges in engagement and retention. The small sample size and heterogeneity limit the generalizability of these findings, but the absence of serious adverse events is notable given the high baseline risk in this group.

Policy, Research, and Regulatory Implications

This trial provides the first direct evidence that PAP is feasible and preliminarily safe for adults with TRD and cluster B PDs, a population typically excluded from psychedelic research due to concerns about emotional instability and risk of adverse reactions. Regulatory agencies such as the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have not yet issued specific guidance for psychedelic trials in comorbid PD populations. The data from NCT05029466 may inform future protocol designs, including risk mitigation strategies, inclusion/exclusion criteria, and monitoring requirements. Notably, the trial's secondary analysis approach highlights the importance of flexible study designs capable of capturing outcomes in complex, real-world populations—a point often overlooked in more homogeneous phase 2 studies.

Risks, Unknowns, and Future Directions

While the absence of serious adverse events is encouraging, the high dropout rate and outcome variability raise important questions about patient selection, therapeutic support, and long-term safety. Cluster B PDs are associated with increased impulsivity and self-harm risk, necessitating robust screening, preparation, and integration protocols. The small sample size precludes definitive conclusions about efficacy or risk, and the open-label design introduces potential bias. Future trials should prioritize larger, more diverse samples, standardized diagnostic assessments, and longer-term follow-up to assess both benefits and risks. Additionally, the trial's experience suggests that tailored psychotherapy protocols—potentially with enhanced support for emotional regulation—may be required for this population.

Looking Ahead: Implications for Practice and Research

This early evidence supports the feasibility of PAP in individuals with TRD and cluster B PDs, but also highlights the need for careful patient selection, specialized therapeutic frameworks, and transparent reporting of both positive and negative outcomes. As the field moves toward larger, multisite trials, collaboration between academic centers, regulators, and patient advocacy groups will be essential to ensure that research addresses the needs of those with the fewest effective options. The trial’s real-world complexity offers a valuable lesson: future psychedelic research must grapple with heterogeneity and risk, not just efficacy, to inform clinical and regulatory decision-making.

By Dr. Alex Morgan, PhD (Neuroscience, Clinical Trials Methodology). Reviewed by Dr. Lisa Chen, MD (Psychiatry, Personality Disorders) on 2026-09-17. We review primary trial registries and published protocols for accuracy and context.