Many people will tell you that marijuana is less harmful than alcohol, but a recent study calls this assertion into question. According to a report from Medical News Today, research published in the journal Psychological Science suggests that smoking high-potency marijuana is linked to a significant change in a key brain structure.

The study, led by senior researcher Dr. Paola Dazzan from the Institute of Psychiatry, Psychology & Neuroscience at King’s College in London, showed that heavy users of cannabis with a high concentration of THC, the main psychoactive ingredient in marijuana, were the most likely to show evidence of brain damage.

The study examined 54 individuals who had experienced at least one episode of psychosis, as well as 43 individuals who were given a clean bill of health. The two groups provided information to the researchers about their drug use history by filling out the Cannabis Experience Questionnaire. In addition to asking about their history of cannabis use, the survey also asked about stimulants and other recreational substances.

The participants were given MRI scans using a technique called diffusion tensor imaging. This allowed researchers to get a detailed view of the brain structures in each of the participants.

The team was particularly interested in a region of the brain called the corpus callosum. Made primarily of white matter, the structure connects the right and left hemispheres in the brain, providing connectivity between the two sides.

The study found that participants who historically used high-potency cannabis on a frequent basis were more likely to have a higher mean-diffusivity in the corpus callosum. This measure is a marker of white matter damage, and there was a strong correlation between heavy cannabis users and high mean-diffusivity in the corpus callosum.

The study suggests that marijuana might not be as harmless as many people believe. Heavy and frequent users of high-potency cannabis run the risk of damaging the white matter fibers in the corpus callosum, which could potentially lead to a large number of cognitive issues down the line.

A press release from King’s College outlining the details of the study can be found here.