Smoking cannabis once as a teen can change your brain – Expert Reaction

Teens who say they have only smoked cannabis once or twice have both structural and cognitive changes in their brains, compared with teens who have never smoked dope, according to a small Australian-led study.

The researchers used brain scans of 46 14-year-olds and found a greater volume of grey matter in some brain regions among the teens with just one or two instances of cannabis use compared with those who had never tried the drug. The researchers also found a link between these changes in grey matter and assessments of reasoning and anxiety.

Our colleagues at the Australian and UK Science Media Centres gathered the following comments on the findings. Please feel free to use these in your reporting:

Dr Liz Temple, Senior Lecturer in Psychology at the University of New England, comments:

“While many past survey-based studies have found that using cannabis before the age of 16 is linked to a range of adverse outcomes, this new research by Catherine Orr and colleagues increases our understanding of how and why this may occur.  In particular, it provides biological evidence that, even when used only once or twice, cannabis use can adversely impact the brains of young teenagers.

It is, however, important to note that the study found only a small number of cognitive functions and behaviours to be associated with these cannabis use-related brain alterations, with many of the assessed cognitive functions and behaviours being unaffected.

It is also important, as noted by the researchers, that further research is conducted to ensure findings can be replicated in larger and more diverse samples.

More in-depth investigation of the concurrent use of cannabis, alcohol and nicotine on the adolescent brain is also important, to increase our understanding of the individual and combined impacts of these substances on normal brain development and function.”

No conflicts of interest declared.

Dr Bernie Cocks, Lecturer in Cognitive Neuroscience, University of New England, comments:

“The results of this study suggest that even in small doses, cannabis use can result in structural brain changes, most notably increased grey matter volume.

“Given that adolescence is a time of rapid brain maturation/reorganisation, any changes which are induced by external forces such as cannabis or alcohol use rather than normal developmental factors must be viewed with some concern.

“The study also noted a large degree of overlap between low-dose users and non-users, suggesting that some individuals may be more at risk than others, although it is not yet possible to identify those at-risk.

“As a consequence, the overall results suggest that cannabis, like alcohol and other recreational drugs, should be avoided until brain development/maturation is complete; that is, until at least the age of 21.

“Paradoxically, the finding that low-dose cannabis use can increase grey matter volume may have therapeutic implications.  Previous studies have suggested that some individuals suffering from some disorders (e.g. major depressive disorder) have reduced grey matter volume, thus, low dose  cannabis use, under medical supervision, may have the potential to reverse this reduction, although a significant amount of research needs to be conducted before such a conclusion can be verified either way.”

No conflicts of interest declared.

Professor Mathew Martin-Iverson, Head of Pharmacology, University of Western Australia, comments:

“This is an interesting finding of increased gray matter volume in the brains of young people who tried cannabis once or twice when they were 14. The data are from a small subset of a much larger multi-centre study that was focused on different issues.

“The findings are an association, and the direction of cause cannot be determined by this study.

“It may be other factors, such as the age at which nicotine or alcohol were first tried, or other personality or environmental characteristics that were not reported in the study that may lead to early experimentation that are causal to the gray matter volume changes, if the effect is real.

“Note that the same effect was not apparent in 16-year-olds who tried cannabis once or twice. The sample sizes are relatively limited and the relative low rate of replication of the findings means that we need replication in other samples before we can be confident that the association exists.

“Furthermore, the significance levels were not corrected for the large number of comparisons made, which has led to positive findings in MR brain imaging in a dead salmon brain.

“It is also unclear what significance an increase in gray matter volume may have. Is it good or bad? In the past, decreases in gray matter volume were associated with alcoholism and schizophrenia, for example.

“This study raises more questions than it answers at this stage.”

No conflicts of interest declared.

Professor Derek Hill, Professor of Medical Imaging, University College London, comments:

“This research uses well established brain image analysis methods to assess whether the size and shape of adolescent brains is changed by using cannabis just once or twice.  Previous work has focused on brain changes in people who are long term heavy users of cannabis, so this research potentially helps understand whether brain changes start after smoking just one or two joints.

“The results suggest that many parts of the brain appear larger in the teenagers who have used cannabis just once or twice.  Slightly bigger brains, however, doesn’t necessarily mean damage to the brain.  There could, for example, be a change in brain volume because of a small change in the amount of fluid between brain cells rather than because any change in the brain cells themselves.

“Furthermore, it is also possible that the larger brains is a random finding. Only 46 teenagers were studied, and the majority of these had just one brain scan, which is compared to a control. The subjects didn’t all have a scan before and after they used the cannabis, which would be a more reliable way of determining whether the cannabis use resulted in a change in their brain.

“The researchers do, however, suggest that their results support evidence from research on animals that has shown that cannabis use can result in changes in the brain cells themselves.  Such a suggestion is an interesting theory, which merits further investigation. But the results in this paper certainly don’t prove that the brain of the teenagers who’ve used cannabis just once or twice are permanently altered in a harmful way.”

 No conflicts of interest.

Professor Sir Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry Psychology & Neuroscience, King’s College London, comments:

“The question of whether cannabis use causes brain changes is an extremely important one given the worldwide increase in use of cannabis.  Previous studies of brain structure have been contradictory with some suggesting that heavy use is associated with decreased brain volumes, some no effect, and some the opposite.

“This study is novel in that it compared 46 adolescents who had used cannabis only once or twice with others who had never used cannabis.  The results show increased cortical volumes in a number of areas such as the temporal lobes (where there are lots cannabinoid receptors). The authors have done their best to rule out other possible explanations e.g. that the differences were present before the cannabis use or the subjects were also using alcohol). This is a sophisticated and well-presented study by an internationally re-knowned team. However, it remains a small study and it is very surprising that persistent brain changes could result from the use of cannabis (or any other recreational drug) only once or twice.  Therefore, the findings will need to be replicated on a much larger scale before we can accept the conclusions.”

 No conflicts of interest.

Professor David Nutt, The Edmond J Safra Chair and Head of the Centre for Neuropsychopharmacology, Imperial College London, comments:

“This is an observational study of brain function and grey matter volume in teenagers who have use cannabis.  They unexpectedly found larger brain volume in some brain regions.

“The imaging aspects of the study have been well conducted and are of good size but ultimately despite using a control group they cannot prove that the differences are due to cannabis use. Their rather imprecise use of the term increased/increases in relation to their brain volume measures incorrectly implies causality.

“Of course it is not possible to do a randomised administration of cannabis to young people to properly test this theory. But if it was a direct pharmacological effect of cannabis then it would likely be more apparent in those using more of it. So it is surprising that they don’t supply data on the brain measures in those who have used cannabis on more than two occasions. If there was a dose effect this would help us understand if this a pharmacological effect of cannabis use.

“Also this increase in size doesn’t seem to have a major impact on brain functioning.  So while this study alone is not able to prove small amounts of cannabis negatively affect the brains of adolescents, this area of research is important and certainly worthy of further study – along with alcohol and other psychoactive substances – as to whether they have unwanted brain effects in young people.’

No conflicts of interest declared.