"Cannabis 'more harmful than alcohol' for teen brains," reports BBC News.
Canadian researchers investigated the links between alcohol and cannabis use and performance on various tests of brain function in 3,826 school pupils over 4 years, starting at age 12 to 13. They found teens who used cannabis performed less well, especially in tests of memory and impulse control.
The researchers said teenagers with worse memory and impulse control were more likely than other teens to use alcohol and cannabis. However, an increase in cannabis use in 1 year was associated with lower test scores that year and also in the following year, suggesting that cannabis could have a lasting effect on their brain function. The researchers did not find this result in teens using alcohol.
Cannabis also seemed to have greater effects in younger teens compared with older teens.
This study will add to the body of research assessing the potential cognitive and mental health effects of cannabis. As is often the case with such research, it's difficult to determine whether cannabis directly causes these effects or whether people at risk of cognitive or mental health problems may be more likely to use cannabis.
We do know that the brains of teenagers are still developing, so any drug or substance that can affect the brain, be it illegal or legal, could possibly have long-term effects.
There are also a number of health risks relating to cannabis use. Read more about how cannabis can lead to health problems.
The researchers who carried out the study were from the University of Montreal, Centre Hospitalier Universitaire Sainte-Justine and Dalhousie University in Canada. It was funded by the Canadian Institutes of Health and published in the peer-reviewed journal The American Journal of Psychiatry.
The study was widely reported in the UK media. The media reports were broadly accurate but the headlines tended towards scaremongering. The Sun referred to cannabis as "brain rot" while the Mail Online implied the results lasted into adult life – which we do not know, as students were not followed up beyond age 16 to 17.
The researchers carried out a cohort study using data from a previously reported randomised controlled trial (RCT). The RCT aimed to assess the effect of a personality-targeted drugs and alcohol prevention programme. It assigned schools to deliver the programme to adolescents (average age 13) either immediately, or 3 years later (a delayed intervention condition).
Cohort studies are useful when investigating links between risk factors such as drug and alcohol use, and outcomes like poor brain functioning. However, it remains a challenge to show that alcohol or drug use is a direct cause of poor brain function.
The trial recruited 3,826 school pupils in 7th grade (age 12 to 13). Pupils underwent computerised tests of brain function every year in school, and filled in a confidential online questionnaire about their cannabis and alcohol use. They continued in the study for 4 years.
The computerised brain function tests measured:
For this study, the researchers compared the brain function scores to pupils' reported use of alcohol or cannabis. This showed whether pupils with poorer brain function in particular areas were more likely to use cannabis or alcohol, and vice versa. They then looked at how students performed year on year, and how that was linked to their reported alcohol or cannabis intake in that year, and in the previous year. This helped to show whether changes in pupils' substance use predicted changes in their test results.
Researchers took into account pupils' family income, gender, ethnicity and whether they lived with both biological parents.
The researchers reported results separately for cannabis and alcohol.
For cannabis, they said:
The researchers said their results showed a "common vulnerability" to using cannabis and alcohol, among pupils with poorer working memory, perceptual reasoning and inhibition control.
The results also supported "a lasting, or neurotoxic, effect of cannabis" on inhibition control and working memory, meaning that the effects on the brain lasted beyond the period that the pupil was using cannabis.
This complex analysis suggests that cannabis use by teenagers may have an ongoing effect on their brain function, particularly in the areas of:
The main difficulty is that we still don't know with certainty whether teens who used alcohol and cannabis had worse brain function because of substance use, or whether they were more likely to use alcohol and cannabis because of their poorer brain function.
Similarly we can't pull apart the influence of confounding health, lifestyle and environmental factors. We don't have a full picture of how other circumstances in their lives, such as peer groups or the home environment, might affect both drug and alcohol use and brain function and academic performance.
If cannabis is having a direct effect on brain function, we can't tell easily from this study how much of an impact this might be. The differences in test results are not easily understood by non-experts. We don't know, for example, if the teenagers in the study who used cannabis were less likely to achieve educational or vocational qualifications, or go on to academic or professional success.
An additional limitation to be aware of is that drug and alcohol use was self-reported. Although teenagers were told the questionnaire was confidential, some may have been reluctant to answer truthfully.
Overall the study adds to evidence that cannabis is not a risk-free drug, especially for teenagers and young people. So avoiding cannabis use during the teenage years – as with any time of life – seems to be a sensible precaution. This study gives teenagers another reason to think twice about using cannabis.