A single dose of psilocybin, the active ingredient in magic mushrooms, can induce anatomical changes in the brain, according to research among people who took the psychedelic compound for the first time.

Scientists spotted apparent changes in the brain’s structure which were still apparent a month after healthy volunteers took the drug. If confirmed, they may help explain the therapeutic effects that psychedelics can have on anxiety, depression and addiction, researchers said.

Evidence for the changes came from specialised scans that measured the diffusion of water along nerve bundles in the brain. They suggested that some nerve tracts had become denser and more robust after the drug was taken. While the findings are preliminary, the scientists said the opposite was seen in ageing and dementia.

“It’s remarkable to see potential anatomical brain changes one month after a single dose of any drug,” said Prof Robin Carhart-Harris, a neurologist at the University of California, San Francisco, and senior author on the study. “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.”

Scientists have long sought to understand how psychedelics affect the brain and the work has gained fresh impetus in the wake of trials and studies that suggest the compounds could be used to treat a range of mental health disorders. The drugs are thought to help by boosting flexible thinking and allowing people to escape destructive cognitive ruts.

In the latest study, Carhart-Harris and colleagues at Imperial College London explored the “entropic brain effect”, in which neural activity becomes more varied on psychedelics; the impact of the drug on people’s wellbeing alongside any functional or anatomical changes in the brain.

To start, 28 healthy volunteers who had never taken a psychedelic were given 1mg of psilocybin, a small enough dose to be considered a placebo. The scientists then used electroencephalography (EEG) to measure their brain activity via electrodes on the scalp.

The volunteers completed a raft of tests over the next few weeks to measure their wellbeing, depth of psychological insight and the flexibility of their thinking. Meanwhile, functional MRI scans and a technique called diffusion tensor imaging (DTI) monitored their brains.

A month after the placebo, participants took a single 25mg dose of psilocybin, enough to elicit a powerful psychedelic experience. During the trip and for weeks after, the scientists ran the same tests and brain monitoring to see what happened.

Within an hour of taking the drug, EEG revealed a surge in brain entropy, suggesting the brain was processing a greater diversity of information. A month later, DTI scans revealed a drop in diffusion along nerve tracts running from the front to the middle of the brain. That could be caused by pruning of some nerve fibres, or the growth of nerves that are not yet covered with insulating sheaths, though more work is needed to confirm the finding.

Writing in Nature Communications, the researchers describe another key finding. Those who had the largest spike in brain entropy after psilocybin were most likely to report deeper psychological insight and better wellbeing a month later, underlining the link between flexible thinking and improved mental health. “It suggests a psychobiological therapeutic action for psilocybin,” said Carhart-Harris.

Prof Alex Kwan, a neuroscientist at Cornell University in New York, said studies in mice had shown that psychedelics can rewire connections between nerves, a form of “plasticity” that could underlie their therapeutic effects. The big question is whether the same occurs in humans. “This study comes closer than most to addressing that question, by giving evidence of lasting changes in brain structure after psychedelic use,” he said. But while the results were “exciting”, the study involved a small number of people and DTI provides an indirect and limited view of brain connections, he said.