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expert reaction to low levels of cannabis use in adolescents and grey matter volume

Research published in Journal of Neuroscience presents evidence suggesting structural brain and cognitive effects in adolescents following one or two instances of cannabis use.

Dr Amir Englund, Post-doctoral researcher in Psychopharmacology, Institute of Psychiatry Psychology & Neuroscience, King’s College London (IoPPN), said:

“The present study is one of the first studies to explore very low level and early use of cannabis on the brain of teenagers, specifically using cannabis only once or twice. The researchers present results which suggest that the teens who had used cannabis (compared to never-users) once or twice over the 2-year follow-up period showed increases in brain volume in areas which have more of the brains cannabinoid receptors. In this study, greater brain volume was associated with slightly worse IQ performance. Although this is a well-designed study, it is important to note that this is one of the first of its kind, with a relatively small sample size and shows conflicting results compared to previous studies exploring brain changes related to cannabis use. A 2015 study (Weiland et al.) which matched users and non-users according to use of alcohol found no differences in brain structures in both adolescents and adults. Two Dutch studies (Koenders et al. 2016, 2017) which followed young adults over a 3-year period found no evidence that cannabis use impacted brain structure. Lastly, a very recent Australian study of 120 non-users and 141 users were not able to detect differences in cortical thickness, brain surface area or gyrification.

“I welcome this new study as we know early use of cannabis is a risk factor/indicator for future mental health problems and cannabis related problems. However, we need more and larger studies in order to confirm the results of this study.”

Prof Derek Hill, Professor of Medical Imaging, University College London (UCL), said:

“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.”

Prof Sir Robin Murray FMedSci, Professor of Psychiatric Research, Institute of Psychiatry Psychology & Neuroscience, King’s College London (IoPPN), said:

“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.”

Prof David Nutt, The Edmond J Safra Chair and Head of the Centre for Neuropsychopharmacology, Division of Brain Sciences,  Dept of Medicine, Imperial College London, Imperial College London, said:

“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.’

‘Grey Matter Volume Differences Associated with Extremely Low Levels of Cannabis Use in Adolescence’ by Catherine Orr et al. was published in The Journal of Neuroscience at 18:00 UK time on Monday 14th January, which is also when the embargo will lift. 

All our previous output on this subject can be seen at this weblink:

Declared interests

Prof Robin Murray: No conflicts of interest.

Prof Derek Hill: “I have no conflicts of interest related to this work.”

None others received.

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