Cannabis use during adolescence: an interview with Dr Edmund Silins

Dr. Edmund SilinsTHOUGHT LEADERS SERIES...insight from the world’s leading experts

How much is known about the impact cannabis use has on the body?

We know quite a lot about the effects of cannabis on the body and brain but there is still a need to better understand its impact on the health, well-being and development of long term users.  

It’s important to remember that cannabis affects different people in different ways. The short-term effects of using cannabis depend on how much of the drug is used, the environment it’s used in, and the size and mood of the person using it.

The short-term effects of using cannabis typically include a feeling of well-being, talkativeness, drowsiness, loss of inhibitions, decreased nausea, increased appetite, loss of co-ordination, and anxiety and paranoia.

In relation to the longer term effects of the drug, the available evidence points to increased risks of respiratory diseases including cancer, later dependence on the drug, decreased memory and learning abilities, and decreased motivation in areas such as study and work.

Some people, particularly younger people and those with a family history of mental health problems, are generally more susceptible to the mental health effects of cannabis. Frequent cannabis use also appears to increase the risk of psychotic illness and the development of depression.

Does this impact differ depending on the age of the user?

The adverse effects of cannabis tend to depend on how early someone starts using the drug and how much they use. The younger a person starts using cannabis and the more frequently they use the drug the more likely they are to experience cannabis-related harm. This is particularly concerning as mid to late adolescence is the period when the risk for cannabis initiation is greatest.

The formative years of adolescence is a critical developmental period as the developing adolescent brain is very vulnerable to the adverse effects of cannabis use.

Please can you give a brief overview of your recent research into cannabis use in young people?

This study brought data together from three large and long-running longitudinal cohorts. Two were Australian (The Australian Temperament Study, The Victorian Adolescent Health Cohort Study), and one was from New Zealand (The Christchurch Health and Development Study). This provided a large sample of about 4,000 people.

We then looked at five different levels of cannabis use in people aged under 17, from no use to daily use, and seven outcomes in young adulthood at age 25.

This study was different from a more traditional meta-analysis because data were integrated at the participant level instead of at the study level. This type of analysis provided increased statistical power, and would not have been possible to do in the individual cohorts – which is why we combined the data. The increased power meant that we could have more confidence in the results.

How did you decide on which seven developmental outcomes to assess?

The decision to assess these seven outcomes was based on previous research which suggested a link with cannabis use, and the data available in the individual cohorts. For example, although it would have been interesting to assess psychosis, this wasn’t possible as none of the cohorts assessed this outcome.

What were your main findings?

There were three important features to the findings. Firstly, there were strong associations between adolescent cannabis use and the outcomes investigated.

Secondly, there was a dose-response effect, in the sense that the more frequently someone used cannabis the more likely they were to experience harms.

And thirdly, these associations remained even after taking in to account a wide range of other factors (53 in total) which might explain the observed associations.

It is these three features of the findings which provide support for a more direct link between early cannabis use and later harms.

The effects were by far the strongest for adolescents who were daily cannabis users. This group, by the age of 25, were about 3 times more likely to drop out of high school or not hold a degree, about seven times more likely to attempt suicide, 18 times more likely to have been cannabis dependent, and eight times as likely to use other illicit drugs.

Were you surprised by any of these associations?

What surprised me most was the clear evidence of a sliding scale of harm with daily users being worst off. However, for adolescents who used relatively infrequently, for example monthly, the risk of harms was also elevated.

These users were almost two times more likely to drop out of high school or not hold a degree, three times more likely to attempt suicide, four times more likely to have been cannabis dependent, and three times more likely to have used other illicit drugs. Pretty startling figures when you consider that the number of kids using cannabis monthly is much, much greater than the number using daily.

What impact do you think this research will have?

I think the findings are timely given recent moves in some countries to decriminalise or legalise cannabis.  Particularly as it’s been suggested that decriminalisation or legalisation might make cannabis more accessible to young people.

However, as we mention in the paper, despite increased availability of cannabis for medical use in some US states, a study in the US showed no increase in use among young people in those states. Whether this is maintained over time remains to be seen.

Nevertheless, it would be wise that any cannabis legislative reforms be carefully assessed to make sure they decrease adolescent cannabis use and prevent potentially adverse effects.

I think it’s important that the potential advantages of decriminalisation, such as reduced criminality, be weighed up alongside evidence of the drug’s potential harms, in particular for adolescents.

Why do you think adolescents are starting cannabis use at a younger age and more adolescents are using cannabis heavily in some countries?

This is a good question with no straight forward answer. A range of factors influence a person’s decision to use illicit drugs like cannabis. Cultural practices, accessibility of cannabis, peer pressure to use drugs, public perceptions of harm, and an individual’s own views about cannabis are all likely to play a part.

What health and social benefits are likely to come from the prevention of cannabis use?

This study found that adolescent cannabis use had marked effects on later health, wellbeing and achievement. Specifically, early use was associated with academic under-achievement, cannabis dependence, mental health problems, and other drug use. These kinds of adverse outcomes can have a substantial impact not only on the current functioning of young people but on their future life options in terms of employment and life satisfaction.

Preventing or at the very least delaying cannabis use in adolescence is likely to improve young peoples’ functioning in these domains and lead to broader benefits for the individual, their family, and the wider community.

What are your further research plans?

Currently the research group is applying the methodology used in the study of early cannabis use to investigate the predictors and developmental consequences of high risk binge drinking in teenagers. But we are bringing data together from four longitudinal cohorts instead of three.

It’s important work because mapping excessive teen drinking patterns to later outcomes is central to understanding the longer term risk status of different drinking behaviours.

Where can readers find more information?

More information about the cannabis study can be found at:

The study authors are part of the Cannabis Cohorts Research Consortium (CCRC), a collaboration which stemmed from the need to better address questions about the relationship between cannabis, other drug use, life-course outcomes and mental health in children and young adults. More information about the CCRC and its work can be found at:

About Dr Edmund Silins

Edmund Silins is an early career researcher with an interest in cannabis research which addresses health issues relevant to young people. He has been involved in a wide range of drug and alcohol research projects which have included: membership of an Australasian consortium of cohort researchers that plan, conduct and disseminate cannabis-related research; developing an interactive computer based drug prevention program for school-age children; investigating the feasibility of peer-led interventions among drug users; and a process evaluation of a non-medicated detoxification unit. His PhD investigated the patterns and correlates of cannabis use in young adults.

Presently he is a Research Fellow at the National Drug and Alcohol Research Centre at the University of New South Wales, Sydney, Australia. He is currently joint coordinator of the Cannabis Cohorts Research Consortium (CCRC), a collaboration of 18 Australian and international researchers concerned with advancing multi-cohort research.

April Cashin-Garbutt

Written by

April Cashin-Garbutt

April graduated with a first-class honours degree in Natural Sciences from Pembroke College, University of Cambridge. During her time as Editor-in-Chief, News-Medical (2012-2017), she kickstarted the content production process and helped to grow the website readership to over 60 million visitors per year. Through interviewing global thought leaders in medicine and life sciences, including Nobel laureates, April developed a passion for neuroscience and now works at the Sainsbury Wellcome Centre for Neural Circuits and Behaviour, located within UCL.


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  1. Malcolm Kyle Malcolm Kyle Netherlands says:

    Health concerns regarding marijuana tend to come from a self-fueling group of discredited scientists funded by the pharmaceutical, prison, tobacco, and alcohol industries. They push non-peer-reviewed papers, fraught with conjecture and confounding variables, while relying upon reports issued by others in their own group to further support their own grossly misleading research and clearly biased agendas.

    The Duke University (New Zealand) study, the one which claimed that smoking marijuana in your teens leads to a long-term drop in IQ, has since been utterly rebuked by a new paper, published by the Proceedings of the National Academy of Sciences that examined the research and found its methodology to be flawed.

    "...existing research suggests an alternative confounding model based on time-varying effects of socioeconomic status on IQ. A simulation of the confounding model reproduces the reported associations from the [August 2012 study], suggesting that the causal effects estimated in Meier et al. are likely to be overestimates, and that the true effect could be zero".
    —Ole Rogeberg.


    Here is a recent peer-reviewed Study proving that Marijuana is not linked with Long Term Cognitive Impairment:

    Amy M. Schreiner of the Department of Psychology at the University of Central Florida recently led a study that looked at 33 existing meta-analyses of cognitive impairment experienced by heavy cannabis users. Schreiner was unable to provide evidence of long-lasting impairment. Specifically, the participants demonstrated no significant cognitive deficiencies once the intoxication period ended. Additionally, Schreiner found no symptoms of impairment in the individuals who had abstained for 25 days. In conclusion she said, "These results fail to support the idea that heavy cannabis use may result in long-term, persistent effects on neuropsychological functioning."


    Schreiner, A. M., Dunn, M. E. (2012). residual effects of cannabis use on neurocognitive performance after prolonged abstinence: A meta-analysis. Experimental and Clinical Psychopharmacology. Advance online publication. doi: 10.1037/a0029117

  2. Max Wood Max Wood United States says:

    Dr. Silins recommended preventing or delaying adolescent cannabis use but didn't mention dosage reduction utensils (a 25-mg vape toke in a flexible drawtube one-hitter instead of lighting up a 500-mg joint) or replacing "$moking" with vaporization.  "Heavy use" may be mainly caused by the fact that a Joint is easier to hide than a One-Hitter.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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