Experts discover how much cannabis you need to smoke to cause brain damage – and it’s not much

Using cannabis just once in your life can worsen parts of your brain related to learning, problem solving and daydreaming, a study suggests.

Researchers from the University of Oxford compared the brain scans of people who had ever used cannabis and found that it is associated with several changes in brain structure and function later in life.

Cannabis use has increased worldwide, especially in countries where legalization has allowed its use for medical and recreational purposes.

But this has happened without a thorough understanding of the potential long-term effects on the brain, say the researchers of the new study.

The study, published in the journal BMJ Mental Healthwas based on data from almost 16,000 British people.

Participants reported whether they had “ever used cannabis,” with response options and then asked how often, with possible answers ranging from once or twice to more than 100 times.

All participants who answered “yes,” approximately 3,500 people, were categorized as lifetime cannabis users for the purposes of the study.

The ‘yes’ group was further subdivided based on how often they used cannabis, with low use defined as less than 10 times in their lifetime, and high use 11 times or more.

Researchers compared the brains of people who have ever used cannabis and found that it is associated with several changes in brain structure and function later in life

They also told how old they were when they last used the drug.

Participants who reported ‘no’ cannabis use were used as a control group to compare brain scans.

Researchers then took into account a range of factors, such as age, general wealth, smoking and alcohol consumption, blood pressure and obesity, which could potentially influence 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 poorer integrity of the white matter – a part of the brain that is essential for cognitive function.

White matter connects different parts 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 between 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 neuronal connectivity in parts of the brain thought to be active during mind wandering or daydreaming.

These brain areas are also densely packed with cannabinoid receptors, cells that respond to the presence of the drug in the body, the researchers point out.

Neither the duration of cannabis abstinence nor the frequency of cannabis use were strongly associated with the observed findings in brain structure and function.

This means that using the drug, even once, was enough to observe the changes in the brain.

Those who had used cannabis at some point in their lives were found to have poorer white matter integrity – a part of the brain essential for cognitive function

But there were some notable differences between men and women.

While significant cannabis-related changes were observed in six specific brain areas in men, in women they were spread across 24 brain structures and functional areas.

This suggests that cannabis use affects the genders differently, the researchers said.

The experts said their findings require further research and acknowledged that they cannot prove that cannabis caused the observed changes because another, currently unidentified factor, could be responsible.

Researchers acknowledge the limitations of using the UK Biobank, which consists of predominantly healthy white participants, which limits the findings as they may not be replicated in more diverse groups.

The researchers were also unable to pinpoint periods of life that could be critical to cannabis’s impact on the brain, such as adolescence or young adulthood.

The study also relied on participants’ ability to accurately recall the amount or frequency of cannabis use during their lifetime, which may have been inaccurate.

“Our results should be interpreted with careful consideration,” researchers warn.

They added: ‘Additional research is needed to understand the effects of heavy cannabis use in this population, including considerations of potency and related information to inform public policy.’

Previous research has linked cannabis use to heart problems, with people who use it up to a third more likely to have a heart attack or stroke.

Scientists in France, who followed more than 1,300 patients in intensive care, found that people who used cannabis alone had almost twice the risk of cardiovascular disease.

Experts have previously suggested that the active ingredient in cannabis, THC, triggers the body’s fight-or-flight response, increasing heart rate and blood pressure.

According to data from the Office for National Statistics, there were 43 deaths linked to cannabis use in 2023.

By comparison, opioids were responsible for 2,551 deaths, cocaine 1,118 and paracetamol 259 deaths, ONS data shows.