Single Dose of Psilocybin Triggers Lasting Brain Changes in Healthy Volunteers

Researchers at University of California, San Francisco and Imperial College London administered a 25 mg dose of psilocybin to 28 psychedelic-naïve volunteers. Participants underwent electroencephalography (EEG) before the trip, during the acute experience, and at several points afterward. Diffusion tensor imaging (DTI) and functional MRI scans were taken before dosing and again one month later. The design allowed comparison of brain activity, structural connectivity, and self-reported wellbeing across the full study period.

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Interest in classic psychedelics has surged after multiple trials suggested benefits for depression, anxiety, and addiction. The “entropic brain” hypothesis posits that psychedelics increase the diversity of neural signaling, potentially breaking rigid thought patterns. Prior work has shown acute functional changes; this study is among the first to report anatomical alterations persisting a month after a single exposure.

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• Prof. Robin Carhart-Harris – Neurologist, UCSF; senior author, Imperial College London.

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• Prof. Alex Kwan – Neuroscientist, Cornell University, New York.

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• Dr. Taylor Lyons – Neuroscientist, Imperial College London.

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• Participants: 28 healthy adults, never before on a psychedelic.

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• Dose: 25 mg psilocybin (full psychedelic dose) after an initial 1 mg placebo session.

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• EEG Findings: Within an hour of dosing, brain entropy surged, indicating a broader range of neural activity.

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• DTI Findings: One month later, diffusion along front-to-midline tracts decreased, suggesting either pruning of fibers or growth of unmyelinated processes.

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• Psychological Correlates: Participants with the largest entropy spike reported deeper insight the next day and higher wellbeing scores at the one-month follow-up.

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The observed link between transient entropy increases, lasting structural changes, and improved self-reported mental health offers a plausible neurobiological pathway for psilocybin’s therapeutic action. If a single psychedelic experience can remodel brain circuitry, it may explain the durability of symptom relief reported in clinical trials for depression and addiction.

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• Carhart-Harris described the anatomical findings as “remarkable” and suggested they point toward a “psychobiological therapeutic action for psilocybin.” He emphasized that participants also showed “positive psychological changes…including improved wellbeing and mental flexibility.”

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• Kwan noted that the study “comes closer than most to addressing” whether human brains exhibit plasticity after psychedelics, while cautioning that the small sample size and indirect nature of DTI limit definitive conclusions.

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• Lyons highlighted that psilocybin appears to “loosen up stereotyped patterns of brain activity and give people the ability to revise entrenched patterns of thought,” underscoring its relevance for mental-health interventions.

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The trial’s modest cohort restricts statistical power, and participants could often tell when they received the active dose, potentially biasing self-reports. DTI provides an indirect view of white-matter architecture, leaving open whether observed diffusion changes reflect fiber pruning, nascent growth, or methodological artifacts. Additionally, some neuroscientists argue that brain entropy may oversimplify the psychedelic state and require more nuanced metrics.

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Researchers disagree on the interpretation of reduced diffusion: it could indicate pruning of existing fibers or the emergence of new, unmyelinated processes. The study offers no data beyond one month, leaving the durability of structural changes uncertain. A lack of a highly sensitive assay for functional brain alterations post-psilocybin remains a noted gap.

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• “It’s remarkable to see potential anatomical brain changes one month after a single dose of any drug,” — Prof. Robin Carhart-Harris, UCSF

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• “We don’t yet know what these changes mean, but we do note that overall, people showed positive psychological changes in this study, including improved wellbeing and mental flexibility.” — Prof. Robin Carhart-Harris

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• “It suggests a psychobiological therapeutic action for psilocybin,” — Prof. Robin Carhart-Harris

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• “This study comes closer than most to addressing that question, by giving evidence of lasting changes in brain structure after psychedelic use,” — Prof. Alex Kwan, Cornell University

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• “Psilocybin seems to loosen up stereotyped patterns of brain activity and give people the ability to revise entrenched patterns of thought,” — Dr. Taylor Lyons, Imperial College London

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The authors call for larger, double-blind trials with extended follow-up periods and multimodal imaging to confirm structural plasticity. Development of more sensitive functional assays and systematic exploration of dose-response relationships are identified as priorities for translating these neurobiological insights into clinical practice.

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