A single dose of psilocybin - the active ingredient in magic mushrooms - may cause physical changes in the brain that stick around for up to a month, according to researchers at UC San Francisco and Imperial College London. The study, published in Nature Communications, involved 28 healthy adults who had never dabbled in psychedelics before. None had a diagnosed mental health condition, but the findings might explain why psilocybin shows promise for treating depression, anxiety, and addiction.

The researchers focused on brain "entropy" - a fancy term for the variety and unpredictability of neural activity. They found that a bigger spike in entropy during the first minutes and hours after a high dose (25 mg) was linked to greater emotional insight the next day. Participants who reported more insight were also more likely to describe improved well-being one month later.

"Psychedelic means 'psyche-revealing,' or making the psyche visible," said senior author Robin Carhart-Harris, PhD, the Ralph Metzner Distinguished Professor of Neurology at UCSF. "Our data shows that such experiences of psychological insight relate to an entropic quality of brain activity and how both are involved in causing subsequent improvements in mental health. It suggests that the trip - and its correlates in the brain - is a key component of how psychedelic therapy works."

The team used EEG, fMRI, and diffusion tensor imaging (DTI) to measure brain activity, connectivity, and structure. They gave participants a 1 mg placebo dose first, then a 25 mg psychedelic dose weeks later. Within 60 minutes of the 25 mg dose, EEG showed a rise in brain entropy. One month later, DTI revealed denser neural tracts - the opposite of age-related degeneration. The day after the high dose, 27 of 28 participants called it the "single most" unusual state of consciousness they'd ever experienced; the remaining participant ranked it in the top five. Participants also scored higher on cognitive flexibility tests a month later.

"Psilocybin seems to loosen up stereotyped patterns of brain activity and give people the ability to revise entrenched patterns of thought," said first author Taylor Lyons, PhD, a research associate at Imperial College London. "The fact that these changes track with insight and improved well-being is especially exciting."

The findings could help clinicians fine-tune psilocybin doses for mental health treatments - enough entropy for insight, but not so much that you forget your own name. "We already knew psilocybin could be helpful for treating mental illness," Carhart-Harris said. "But now we have a much better understanding of how."