Cannabis 'more harmful than alcohol' for teen brains
Wednesday 3rd October 2018
Teenagers using cannabis are causing long-lasting damage to their developing brains, a Canadian study suggests.
It found the impact on thinking skills, memory and behaviour was worse than that of teenage drinking.
The researchers, from the University of Montreal, urged teenagers to delay their use of cannabis for as long as they felt able.
The study tracked and tested 3,800 adolescents over four years, starting from around the age of 13.
Drinking alcohol and taking drugs, such as cannabis, at a young age is known to cause problems with cognitive abilities such as learning, attention and decision-making as well as academic performance at school.
This study found these problems increased as cannabis use increased - and the effects were lasting, unlike those of alcohol.
What are the health risks of cannabis use?
The teenagers, from 31 different Canadian schools, gave details of their drug and drinking habits once a year.
Their brain skills were also tested every year in school using computer-based cognitive tests.
Although levels of cannabis use in the study were low compared with alcohol use, 28% of the teenagers still admitted to some kind of use.
This compared with 75% of the teenagers who said they drank alcohol at least occasionally.
Prof Patricia J Conrod, lead study author, from the department of psychiatry at Montreal, said she had expected alcohol to have had more of an impact on the teenagers' brains.
But, instead, the research detected greater increases in errors in cognitive tests on the teenagers using cannabis - while they were taking the drug and after they had stopped.
These effects were seen in working memory, reasoning and their ability to control their behaviour.
"Their brains are still developing but cannabis is interfering with that," Prof Conrod said, of teenagers.
"They should delay their use of cannabis as long as they can."
She also said the findings highlighted the importance of drug-prevention programmes.
Cannabis is thought to be the most widely used illegal drug in the UK.
It can be addictive and using it has been shown to increase the risk of developing psychotic illnesses, particularly in adolescents.
Giving up cannabis can lead to withdrawal symptoms, such as sleeping problems and mood swings, experts say.
The study is published in the American Journal of Psychiatry.
Where did the story come from?
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.
What kind of research was this?
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.
What did the research involve?
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:
working memory - the short-term memory that allows you to remember information sufficient to complete tasks
perceptual reasoning - the ability to use information from our senses to understand the world around us
delayed recall memory - the longer-term ability to remember something after a period of distraction
inhibitory control - the ability to control natural impulses, for example not to respond to a stimulus
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.
What were the basic results?
The researchers reported results separately for cannabis and alcohol.
For cannabis, they said:
pupils who used cannabis more frequently over 4 years had poorer results on working memory, perceptual reasoning and inhibition control tests, compared to those who did not use cannabis
pupils who increased how much cannabis they used had poorer results than expected in delayed memory tests in the same year
pupils who increased their cannabis use had poorer results than expected in inhibition control the following year
stronger links were observed in early adolescence compared with later adolescence
pupils who drank more alcohol more often over 4 years had poorer working memory, perceptual reasoning and inhibitory control
pupils' changes in alcohol use over time did not seem to be related to their brain function tests
How did the researchers interpret the results?
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:
working memory (important for completing tasks)
perceptual reasoning (important for understanding the world)
inhibition control (important for learning to resist harmful impulses)
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.