Scientists in Colorado have claimed that using cannabis may make your brain larger in later life — but only if you use the drug sparingly.
In a study on 26,000 people in their 50s, of which about 5,600 had smoked cannabis, researchers found that those who used the drug a ‘moderate’ amount — one to 100 times in their lifetime — had larger brain regions linked to memory and cognition compared to those who had never touched the drug.
Meanwhile, people who had used the drug a ‘high’ amount — more than 100 times in their lifetime — had brain regions that were a similar size to non-users or those who used cannabis in moderate amounts. For two of the 11 brain regions measured, however, these were smaller in these users than in the other two groups.
Patients were interviewed in 2010 and were all from the UK, which did not legalize cannabis until 2018, when it was only approved for medical use to treat a small number of conditions. The drug is illegal for recreational use.
Dr Anika Guha, a neuroscientist at the University of Colorado Anschutz, who led the study, said she was ‘surprised’ by the results that hinted cannabis may have some unrealized benefits.
She added in a press release, however: ‘We did see that for many of our outcome measures, moderation seemed to be best.
‘For the brain regions and cognitive tests that demonstrated an effect, the moderate-use group generally had larger brain volumes and better cognitive performance.’
Previous studies have shown that a better cognitive performance is linked to a lower risk of suffering from dementia later in life. Scientists say it may indicate a higher cognitive reserve, or the brain’s ability to maintain normal function and resist damage from aging.
Jana Nelson was 50 when diagnosed with early onset dementia, following severe personality changes and a sharp cognitive decline that left her unable to solve simple math problems or name colors. There is no suggestion that she developed this complication because of cannabis
Cannabis is the most commonly used federally illegal drug in the US and is estimated to be taken by about 52.5million people, or one in five Americans, at least once a year.
About 18million Americans report using cannabis daily or near-daily, according to the CDC, an 18-fold surge from 1992.
Nationwide, 24 states have legalized marijuana for recreational use, while 40 states allow the drug to be prescribed medically.
In the study, published in the Journal of Studies on Alcohol and Drugs, researchers analyzed data from the UK Biobank, a major database of 500,000 Britons that monitors them and questions them on their habits once every four years.
For the paper, scientists analyzed data on 26,000 participants aged 40 to 77 years old, with an average age of 55, who had submitted MRI scans of their brains.
Of these, 5,300 said they had used cannabis a ‘moderate’ amount, while 633 said they had used cannabis a ‘high’ amount over their lifetime.
A total of 16,000 participants, or 60 percent of the total, had also completed cognitive tests, but the scientists did not specify what proportion of these had used cannabis moderately or to a high level.
Scientists measured 11 distinct brain regions in participants and compared them based on cannabis use. These included the hippocampus, vital for transferring short-term memories into long-term, and the amygdala, a primary hub for processing emotions, particularly fear.
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There are many warnings over using cannabis, particularly for young adults, with researchers saying that the drug can affect how the brain develops (stock image)
Scientists analyzed these regions because they had more cannabinoid receptors which, the researchers said, meant they were most likely to be influenced by the drug.
Overall, the scientists found that, compared to non-users, those who said they used cannabis a moderate amount had eight brain regions that were larger than among those who said they never used cannabis.
These were: The left and right putamen, an area of the brain linked to speech articulation and emotional processing; the right caudate, linked to memory; the left and right hippocampus; the left amygdala; and the left and right anterior parahippocampal gyrus, linked to encoding new memories.
But in this group, researchers also found they had two brain regions that were smaller compared to non-users, the left and right posterior cingulate gyrus, which are linked to self-referential thought, memory and spatial navigation.
For those who used cannabis in high amounts, they were shown to have a larger left and right anterior cingulate gyrus than the other two groups. They also had a larger right amygdala than the other two groups, an area of the brain linked to processing, expressing and recognizing fear-inducing stimuli.
But their left and right posterior cingulate gyrus was also shown to be smaller than in the other two groups.
Guha said it was not the case that a larger brain region meant someone had better brain function, but she said that cognitive tests showed those who used cannabis a moderate amount had better memory and cognition than the other two groups.
She added: ‘I think sometimes people have seen my poster on this project or they see the headline and they say, “Great, I’ll just use more cannabis”.
‘But it’s more complicated than that. I think it depends on how people are using and what outcomes you’re looking at.’
She added: ‘It’s a good reminder that these effects involve multiple processes. It’s not all good or all bad.’
The study did not suggest how cannabis use may boost brain volume, but in previous research, scientists said the drug stimulates neurons and can boost connections, which may affect brain size and function.
Cannabis is becoming easier to access and more potent. The graph refers to smokable cannabis. Other forms, such as vapes, contain up to 98 percent of THC, the psychoactive chemical in the drug linked to psychosis
Other studies have previously suggested that cannabis may help to protect against dementia, including a 2024 paper that found marijuana users experience ‘significantly less cognitive decline’ over their lifetimes than non-users.
But other papers have gone the other way, including a Canadian study from last year that found people who visited the hospital for cannabis-related problems were 72 percent more likely than their peers to develop dementia later in life.
And researchers have repeatedly raised the alarm over cannabis use, especially before the age of 25 years, when the brain is still developing, saying the drug can affect brain development and raise the risk of mental health problems. In extreme cases, it could also lead to psychosis or a loss of touch with reality.
The new study had several limitations, including that it was observational and could not definitively prove whether cannabis use directly led to larger brains in older adults, or if other factors were at play.
The data was also self-reported, meaning patients did not have to provide evidence of how often they used marijuana.
It also did not record how the cannabis was consumed and did not track THC or CBD content in the cannabis used.
In the US, THC content in the drug has surged, rising from about four percent in the 1990s to well over 20 to 30 percent today. In the UK, content in cannabis has also risen, with some strains reaching 35 percent THC.
