For decades, scientists have known that a single high dose of psilocybin — the psychoactive compound in “magic mushrooms”, can leave people feeling better for weeks or months afterward. What they haven’t been able to show is what changes in the brain to make that happen. A new study, led by researchers at Imperial College London and UC San Francisco and published in Nature Communications, is one of the first to track those changes directly, from the first hour of the trip to a full month later.

The setup

The researchers recruited 28 healthy adults who had never taken a psychedelic before, a psychedelically “blank slate” group, which let the team isolate the drug’s effects without the complication of prior use or an existing mental health diagnosis. Each participant received two doses a month apart: first a 1 mg “placebo” dose, too small to produce any noticeable effect, then later a full 25 mg dose capable of producing an intense psychedelic experience. Brain activity was recorded with EEG during both sessions, and MRI scans were taken before and after each one, so the team could compare the real trip against the inactive control within the same person.

A brief flood of “information”

Within the first two hours of the 25 mg dose, EEG recordings showed a sharp rise in what researchers call brain “entropy”, a measure of how varied and unpredictable brain activity is. In simple terms, the psychedelic brain looks less locked into its usual repetitive patterns and more open to novel configurations. Nothing like this appeared after the 1 mg placebo.

That momentary surge in entropy turned out to matter a lot: participants who showed the biggest increases were the ones who reported the greatest sense of psychological insight the very next day, and those insight gains, in turn, predicted better overall well-being a full month later. The effect chained together like a relay: more entropy → more insight → better well-being.

A brain that looks different a month on

The most surprising result came from diffusion tensor imaging (DTI), a scanning technique that tracks how water moves along the brain’s white-matter fibers. A month after the 25 mg dose, but not after the placebo, the researchers found that certain tracts connecting the prefrontal cortex to deeper brain structures had become measurably denser and more compact. That’s notable partly because it’s the opposite of what typically happens with aging, when these same tracts tend to grow more diffuse. The team is cautious about over-interpreting this finding, the underlying biology of the signal is genuinely ambiguous, but it’s the first time this kind of anatomical shift has shown up after a single psychedelic dose in humans.

Participants also performed better on a task of cognitive flexibility a month later, and nearly everyone (27 of 28) rated the 25 mg trip as the single most unusual state of consciousness of their entire lives.

Why it matters

As senior author Robin Carhart-Harris put it: “We already knew psilocybin could be helpful for treating mental illness. But now we have a much better understanding of how.” The findings support a growing idea in psychedelic research, that the subjective trip itself, not just the drug’s pharmacology, is doing real therapeutic work. That has practical implications: if entropy levels during the trip predict outcomes weeks later, clinicians might eventually be able to tailor dosing to make sure a patient reaches the “sweet spot” of brain entropy needed to spark real psychological insight.

The study was small and exploratory, and the participants were healthy rather than clinically depressed or anxious, so it’s not yet clear the same brain changes will show up in patients. But it’s a rare, detailed look at what a single guided psychedelic experience actually does to the human brain, and how that lines up with what people say it does to their minds.

Source: Lyons, T. et al. “Human brain changes after first psilocybin use.” Nature Communications 17, 3977 (2026). DOI: 10.1038/s41467-026-71962-3
https://www.nature.com/articles/s41467-026-71962-3