Psilocybin for Treatment-Resistant Depression: A Comprehensive Review of Mechanisms and Therapeutic Potential

ABSTRACT Treatment-resistant depression (TRD) is characterized by chronic symptoms, impaired functioning, and limited response to conventional antidepressant therapy. Contemporary reviews have highlighted increasing interest in psilocybin-assisted therapy as a mechanistically novel approach for depressive disorders, grounded in early feasibility work demonstrating clinically meaningful symptom reductions in TRD populations and strengthened by subsequent randomized trials in major depressive disorder (MDD).¹,²,³ Open-label findings in TRD showed rapid and sustained reductions in depressive symptoms, while controlled trials in MDD demonstrated significant improvements from baseline after one or two psilocybin-assisted therapy sessions.¹,²,³ Systematic reviews of mechanistic work indicate that psilocybin modulates multiple neurobiological and psychological domains relevant to depression, including 5-HT₂A-mediated signaling functional network reorganization and emotional or cognitive shifts associated with therapeutic processes such as openness and acceptance.²,⁴‾⁷ Neuroimaging studies show acute alterations in default mode network dynamics and broader changes in connectivity during the psychedelic experience, findings interpreted as consistent with increased cognitive and emotional flexibility.²,⁵,⁷ Meta-analytic reviews report substantial antidepressant effects across different psilocybin dosing strategies and indicate that improvements often persist for several weeks.⁸,⁹ Safety reviews describe psilocybin’s acute adverse effects as generally mild, transient, and self-limiting, whereas recent systematic analyses highlight substantial variability and methodological limitations in harms reporting across trials.¹⁰,¹¹ Ethical, legal, and implementation challenges remain, including the constraints imposed by psilocybin’s Schedule I classification and the intensive training, therapeutic support, and clinical infrastructure required for safe delivery of psilocybin-assisted therapy.¹²,¹³ This narrative review synthesizes mechanistic, clinical, safety, and regulatory evidence to evaluate the potential role of psilocybin-assisted therapy for individuals with TRD.