In brief
- Twenty-eight people with no prior psychedelic experience received 25 mg of psilocybin and were studied using electroencephalography, functional MRI, and diffusion imaging.
- EEG entropy increased during the first hours; one-month findings concerned diffusion, not a measure of persistently elevated entropy.
- The authors themselves urge caution: the sample size is small, and the biological significance of these structural changes remains unclear.
A team from the University of California, San Francisco, and Imperial College London has published an exploratory multimodal study in Nature Communications regarding what remains in the brain after the acute effects of psilocybin have worn off. The short answer: something measurable, even a month later.
How it was done
The study involved 28 healthy adults who had never taken a psychedelic—a criterion that reduces that prior exposure without eliminating expectations or selection bias. Each person first received 1 mg of psilocybin, a dose so low it functioned as a control, followed one month later by a high dose of 25 mg. The modalities had different schedules: EEG before and at one, two and 4.5 hours during dosing; functional MRI and diffusion before and at one month. EEG was not measured at thirty days.
The article describes intense subjective experiences with 25 mg. Intensity is not a measure of clinical benefit, and an item-specific self-report denominator is not recalculated here.
Acute and one-month findings
EEG entropy increased at one and two hours after 25 mg, not a verified month-long persistence. The abstract describes an association of that acute measure with later wellbeing and of next-day insight with that relationship; these are exploratory analyses, not proof of causal mediation in an individual. It also reports changes in cognitive flexibility, personal insight and wellbeing at one month.
Diffusion imaging showed lower axial diffusivity in prefrontal-subcortical tracts at one month, associated with changes in functional modularity. This is an indirect measure and does not certify healthier or more intact fibres, new neurons, repair or reversal of ageing. The abstract says enduring functional changes were largely absent.
What it implies
There are reasons for interest and reasons to temper expectations. In favor: the study uses a design with three neuroimaging modalities, involves people with no prior exposure, and includes a thirty-day follow-up, which is rare. Against: twenty-eight participants is a small sample, all were healthy, and the order of the sessions was always the same—low dose first, high dose later—so the effect of expectations or test repetition cannot be entirely ruled out. The authors also emphasize that more studies are needed to understand what these structural changes actually mean; diffusion is an indirect measure and is not equivalent to having observed new neurons.
This work fits with other recent pieces on the common brain pattern of psychedelics and provides something that was missing: a long time window. If you want the general context of this substance, we have gathered it in our Psychedelics Guide on psilocybin and magic mushrooms.
Source
- Lyons T, Spriggs M, Kerkelä L et al. Human brain changes after first psilocybin use. Nature Communications 17(1):3977, May 5, 2026. DOI 10.1038/s41467-026-71962-3.
Educational content written from a harm reduction perspective and with respect for individual freedom. It is not a substitute for advice from a healthcare professional and is not intended to encourage or condemn any drug use.
