Using cannabis just once in your life could cause parts of your brain associated with learning, problem solving and daydreaming to deteriorate, a study suggests.

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

Cannabis use has increased worldwide especially in countries where legalisation has permitted its use for medical and recreational purposes. 

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

The study, published in the journal BMJ Mental Health, drew on data on almost 16,000 Brits.

Participants reported if they had ‘ever taken cannabis’, with response options then asking them how often with possible answers ranging from once or twice to more than 100 times.

All participants who responded ‘yes’, about 3,500 people, were categorised as lifetime cannabis users for the purposes of the study.

The 'yes' group was further divided into by how often they used cannabis, with the low use defined as less than 10-times in their lifetime, and high use 11 or more times. 

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

Participants that reported 'no' cannabis use were used as the control group to compare brain scans to. 

Researchers then accounted for 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 had ever used cannabis in their lifetime were found to have a poorer white matter integrity — a part of the brain vital for cognitive function.

White matter connects different regions of the brain playing an important role in learning and problem solving as well as balance and walking. 

But researchers found using cannabis reduces how well this part of the brain communicates, 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 areas of the brain are also densely packed with cannabinoid receptors, cells that reacts 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 any of the observed findings in brain structure and function.

This means using the drug even once was enough for the changes in the brain to be observed.  

But there were some notable differences between men and women. 

While significant cannabis related changes were observed in six specific brain regions among the men, in women these were spread across 24 brain structures and functional regions.

This suggests cannabis use affects the sexes differently, the researchers said.

The experts said their findings need further research and acknowledged that they cannot prove cannabis caused the observed changes as some other, currently unidentified factor, could be responsible. 

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 replicated in more diverse groups. 

The researchers were also unable to pinpoint any periods during life that might be critical for cannabis effecting 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 need to 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 take it up to a third more likely to suffer a heart attack or stroke. 

Scientists in France, who tracked more than 1,300 patients in intensive care, found people who took cannabis alone had almost twice the risk of suffering a cardiovascular event. 

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

In 2023 there were 43 deaths linked with cannabis use, according to data from Office for National Statistics. 

In comparison, opioids accounted for 2551 deaths, cocaine 1118 and paracetamol 259 deaths, ONS data shows.