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expert reaction to cannabis and potential brain abnormalities

Researchers in the US found a correlation between casual use of cannabis and the size, shape, and structure of brain regions involved with motivation, emotion and reward in a small sample of young adults. The report was published in the Journal of Neuroscience.


Prof Terrie Moffitt, Professor of Social Behaviour and Development, King’s College London’s Institute of Psychiatry, said:

“This is a new paper by Gilman et al reporting brain structure abnormalities among 20 young adult cannabis users who use cannabis casually and are not dependent. The finding is not in keeping with findings from my team’s 2012 report in PNAS from the Dunedin longitudinal cohort study. There, we conducted neuropsychological testing of 1000 individuals as children, before any of them used cannabis, and then followed them to age 38, when we repeated the neuropsychological testing. Thus, we were able to study change in the function of the participants’ brains over the years. 

“What differed from the Gilman report? We found that brain function had declined among cannabis users, but only among individuals whose cannabis use began in the teen years, and continued at a fairly heavy rate over the subsequent years during adulthood (at least 4 days a week), or those who became dependent on cannabis. In other words, we did not find any decline in the brain’s functions among casual, short-term cannabis users. Our study had 479 individuals who were casual cannabis users, and represented casual users in their country.

“We could wish that the Gilman et al study had a more informative design.

“As one example, it is not clear from the paper where the 20 cannabis users came from, or where the controls came from, nor how well they represent cannabis users in the real world. MRI research is expensive, which keeps samples small, but small samples prevent us from extrapolating findings to cannabis users in the population.

“The text of the paper refers to brain “changes,” but the paper says there was only one MRI session.  Comparing pre-cannabis MRI to post-cannabis MRI is required to document change.

“The paper shows group differences in brain structure, but unless these differences are accompanied by evidence of deficits in brain function (such as thinking, memory, attention), it is unclear whether the findings have implications for the debate about the safety of cannabis. The study could have tested for brain functions, or even interviewed the participants about their self-perceived cognitive complaints, but this key information was missing.”


Prof Peter Jones, Professor of Psychiatry, University of Cambridge, said:

“This is an interesting piece of research where the authors have scanned the brains of students who had or hadn’t used cannabis. However, the research is limited as it is only a small study, it is not known whether the reported changes in the brain are necessarily bad (the main example being an enlarged nucleus accumbens) and nor is it known how significant the amount of the active ingredient THC is (it was not measured in the study).

“Furthermore, there were only 40 people in the study and as they didn’t measure the brains before and after, it’s possible that people with a larger accumbens are more likely to take cannabis.

“The main point is that, as usual, more research is needed.”



Dr Michael Bloomfield, MRC Clinical Research Fellow & Honorary Specialty Registrar in Psychiatry at the MRC Clinical Sciences Centre, said:

“It’s been known for some time that heavy cannabis use can affect the brain.  This new, well-conducted study has some really interesting and important results because it suggests that even moderate use of cannabis in young adults may be associated with changes in the brain. This study found that cannabis use was associated with changes in two key brain regions: the nucleus accumbens, which processes information about pleasure and can be involved in addictive behaviours, and the amygdala, which is involved in processing emotions including fear.

“This study found clues that the changes in these brain regions may be related to current cannabis use. This type of study, alone, can’t prove that cannabis use is causing these changes. It might, for example, be that people who have a difference in their nucleus accumbens or amygdala may be more likely to smoke cannabis.  To show that cannabis use is causing the changes would require looking at brain scans in the same people over a period of time. 

“Previous research has shown that heavy cannabis use in adolescence is associated with changes in how the brain is connected and our own research at the MRC Clinical Sciences Centre in London has found that regular cannabis use in young adults seems to lower the brain’s ability to make the chemical dopamine. 

“Taken together, these studies therefore have implications for understanding some of the mental health problems that are associated with cannabis use including schizophrenia, particularly as the younger people are when they use start using cannabis, the higher the risk of mental illnesses down the line.”


‘Cannabis Use is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users’ by Gilman et al. published in Journal of Neuroscience on Tuesday 15th April.

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