Almost two thirds of people who regularly use cannabis are likely to suffer issues with brain function, concerning research today suggested.
Advocates of the drug often purport that marijuana is safe because it is ‘natural’.
But US scientists, who tracked over 1,000 young adults, found 63 per cent of heavy cannabis users had reduced brain activity during memory tasks.
The figure stood at almost seven in 10 among recent users.
Experts, who claimed it was the largest study of its kind, urged users to reflect on the risk of consistent cannabis use but warned further, longer research was still vital.
In the study, 88 participants were classed as heavy cannabis users, with 179 moderate users who were then compared to 736 non users.
Heavy users were considered those who had used cannabis more than 1,000 times over their lifetime.
Researchers studied their brain activity via MRI scans as all participants undertook seven different tasks.
US scientists, who tracked over 1,000 young adults, found 63 per cent of heavy cannabis users had reduced brain activity, during memory tasks
These tested working memory, emotional response, language, motor skills – with activities including tasks such as tapping a finger to map brain control.
The researchers found that cannabis had a statistically significant effect on brain function during working memory tasks, with 63 per cent of heavy users showing reduced brain activity.
This, the scientists said, was ‘very unlikely to be due to random chance’.
During working memory tasks, the researchers also said that heavy users appeared to have reduce brain activity in specific areas of the brain — the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex and anterior insula.
These regions are involved in important cognitive functions such as decision-making, memory, attention and emotional processing.
A similar effect on working memory was seen in moderate cannabis users. But the impact was less significant across the other tasks.
Writing in the journal JAMA Network Open, the researchers said: ‘Our findings highlight the need to educate cannabis users about the consequences of recent and heavy lifetime cannabis use on cognitively demanding working memory tasks.
‘Similarly, the association between heavy use and decreased brain function could motivate regular cannabis users to reduce their cannabis use and could encourage treatment.’
In the UK, laws around the medical use of cannabis were relaxed almost seven years ago. It is available on the NHS under strict guidelines to help control seizures, for chronic pain and to those with the degenerative condition multiple sclerosis
Dr Joshua Gowin, an assistant professor in radiology at the University of Colorado and study co-author also said: ‘As cannabis use continues to grow globally, studying its effects on human health has become increasingly important.
‘By doing so, we can provide a well-rounded understanding of both the benefits and risks of cannabis use, empowering people to make informed decisions and fully comprehend the potential consequences.’
He added: ‘While some of the other tasks indicated potential cognitive impairment, only the working memory task showed a statistically significant impact.
‘People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well.’
However, Dr Gowin said there still unanswered questions about how cannabis use impacts the brain, and larger, and longer term studies particularly on how long the effects last and if they hit different age groups harder were needed.
It comes as NHS figures earlier this month found a record number of older Brits were hospitalised after smoking cannabis last year amid a surge in ‘silver smokers’.
More than 2,700 adults aged 65 and older — including 9 over 90s — were admitted to hospital in England between April 2023 and March 2024 suffering adverse reactions or overdoses.
It marks a rise of 56 per cent on pre-pandemic levels and a staggering 650 per cent on a decade ago.
Patients reported experiencing hallucinations, withdrawal symptoms and even suffering cannabis poisoning.
Separate annual figures from the Office for National Statistics show cannabis remains the most popular drug among 16 to-59-year-olds in England and Wales.
Use was highest among 20 to 24-year-olds last year, official figures show.
Just over a quarter of all young adults quizzed between April 2023 and March 2024 said they had ever smoked weed in their lifetime.
In the UK, laws around the medical use of cannabis were relaxed almost seven years ago.
It is available on the NHS under strict guidelines to help control seizures, for chronic pain and to those with the degenerative condition multiple sclerosis — however, just 110 prescriptions have been issued in the past five years.
Last year, private cannabis prescriptions doubled — reaching nearly 180,000 — according to the Care Quality Commission (CQC).
Meanwhile in the US, the drug has seen a meteoric rise over the past decade, culminating in legal recreational use in 24 states.
THC — one of the main active components found in marijuana — stimulates areas of the brain involved with mood, attention and memory, while triggering the release of the hormone dopamine, responsible for feelings of reward and pleasure.
Small, infrequent doses have little long-term impact, according to studies.
But with prolonged, regular use, signals in these key brain areas can start to go awry.
Studies have shown that frequent cannabis use can increase the risk of serious mental illness like schizophrenia, as well as insomnia, social anxiety disorder and suicidal thoughts.
