Australasian Science: Australia's authority on science since 1938

Smoking Dope Just Once as a Teen Could Change Your Brain

By AusSMC

Teenagers who say they have only smoked cannabis once or twice have both structural and cognitive changes to their brains.

“The results of this study (Journal of Neuroscience, https://goo.gl/CtMzvL) 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.”

Dr Bernie Cocks is a lecturer in cognitive neuroscience at the University of New England.


“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.”

Dr Liz Temple is a Senior Lecturer in Psychology at the University of New England.


“This is an interesting finding of increased grey 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 grey 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.

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

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

Prof Mathew Martin-Iverson is Head of Pharmacology at the University of Western Australia.