Using cannabis just once in a lifetime could impair parts of the brain associated with learning, problem-solving and dreaming, a study suggests.
Researchers at the University of Oxford compared brain scans of people who had ever used cannabis and found that it is associated with several changes in brain structure and function in old age.
Cannabis use has increased around the world, especially in countries where legalization has allowed its use for medical and recreational purposes.
But this has happened without a deep understanding of its possible long-term effects on the brain, the researchers of the new study say.
The study, published in the journal BMJ Mental HealthIt was based on data from almost 16,000 Britons.
Participants reported whether they had “ever used cannabis,” with response options, and then were asked how often; Possible answers ranged from one or two to more than 100 times.
All participants who answered “yes,” about 3,500 people, were classified as lifetime cannabis users for the purposes of the study.
The group that answered “yes” was divided by how often they used cannabis: low use was defined as fewer than 10 times in their lifetime and high use was defined as 11 or more times in their lifetime.
Researchers compared the brains of people who once used cannabis and found that it is associated with several changes in brain structure and function in old age.
They also reported how old they were the last time they used the drug.
Participants who reported “no” cannabis use were used as a control group to compare brain scans.
The researchers then took into account a variety of factors such as age, overall wealth, smoking and alcohol consumption, blood pressure and obesity, which could potentially influence the brain scans.
Even after adjusting for these factors, cannabis use was associated with multiple changes in brain structure and function.
Those who had used cannabis at some point in their lives were found to have worse white matter integrity, a part of the brain vital for cognitive function.
White matter connects different regions of the brain and plays an important role in learning and problem solving, as well as balance and walking.
But researchers found that cannabis use reduces communication in this part of the brain, especially between the right and left sides of the organ.
Cannabis users were also found to have weaker resting-state neural connectivity in parts of the brain thought to be active during mind-wandering or daydreaming.
These areas of the brain are also packed with cannabinoid receptors, cells that react to the presence of the drug in the body, the researchers note.
Neither duration of cannabis abstinence nor frequency of cannabis use was strongly associated with any of the observed findings in brain structure and function.
This means that using the drug even once was enough for changes in the brain to be observed.
Those who had ever used cannabis in their lives were found to have worse white matter integrity, a part of the brain that is vital for cognitive function.
But there were some notable differences between men and women.
While significant cannabis-related changes were observed in six specific brain regions among men, in women these were distributed across 24 brain structures and functional regions.
This suggests that cannabis use affects the sexes differently, the researchers said.
Experts said their findings need further research and acknowledged that they cannot prove that cannabis caused the observed changes, as some other, currently unidentified, factor could be responsible.
The researchers acknowledge the limitations of using the UK Biobank, which is made up of predominantly healthy white participants, which limits the findings as they may not be able to be replicated in more diverse groups.
The researchers were also unable to identify any period during life that might be critical for cannabis to affect the brain, such as adolescence or early adulthood.
The study also relied on participants’ ability to accurately recall the amount or frequency of their lifetime cannabis use, which may have been inaccurate.
“Our results should be interpreted with careful consideration,” the researchers caution.
They added: “Additional research is needed to understand the effects of excessive cannabis use in this population, including potency considerations and related information to inform public policy.”
Previous research has linked cannabis use to heart problems, and people who use it are up to a third more likely to suffer a heart attack or stroke.
Scientists in France, who followed more than 1,300 patients in intensive care, found that people who took cannabis alone had almost double the risk of suffering a cardiovascular event.
Experts have previously suggested that cannabis’s active ingredient, THC, triggers the body’s fight-or-flight response, increasing heart rate and blood pressure.
In 2023 there were 43 deaths related to cannabis use, according to data from the Office for National Statistics.
By comparison, opioids accounted for 2,551 deaths, cocaine 1,118 and paracetamol 259, ONS data shows.