Teen cannabis use and poor life outcomes – experts respond

Frequent teenage cannabis use is linked with less educational achievement and other problems later in life, according to new New Zealand and Australian research.

File:Marijuana.jpgIndividuals who are daily users of cannabis before age 17 are less likely to complete high school or obtain a degree compared with those who have never used the drug, new research published in The Lancet Psychiatry journal shows.

The study also indicates that, compared to people who never used cannabis in their youth, daily users of cannabis during adolescence are seven times more likely to attempt suicide, have an 18 times greater chance of cannabis dependence, and are eight times as likely to use other illicit drugs by age 30.

The research combined data from the long running University of Otago Christchurch Health and Development Study with two similar studies in Australia, covering more than 3700 participants enrolled in ‘cohort’ studies which followed the same individuals for over thirty years.

Co-author Prof David Fergusson from the University of Otago, Christchurch, said in a media release that these associations are consistent across studies, and cannot be explained by differences in the family background or childhood experience of cannabis users and non-users.

According to the study’s lead author, Dr Edmund Silins, “Our results provide strong evidence that the prevention or delay of cannabis use is likely to have broad health and social benefits. Efforts to reform cannabis legislation should be carefully assessed to ensure they reduce adolescent cannabis use and prevent potentially  adverse effects on adolescent development.”

New Zealand and Australia have some of the the highest rates of cannabis consumption in the OECD and some studies estimate that by the age of 21, about 80 per cent of young New Zealanders will have used cannabis on at least one occasion.

The SMC collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; smc@sciencemediacentre.co.nz).

Associate Prof Simon Denny, Faculty of Medical and Health Sciences, University of Auckland, comments:

“Overall this paper shows that increasing frequency of cannabis use increases the risk of poor outcomes in adulthood. This paper provides the best evidence to date on the harms of marijuana use during adolescence as it combines three large longitudinal studies that track the lives of children through to adulthood. However as it is not a randomised study,  there is still the possibility that adolescents who use marijuana have other aspects of their lives that make them more susceptible to poor outcomes during adulthood. Especially adolescents who use marijuana frequently, these young people are obviously in different family environments from adolescents whose families don’t allow these behaviours.

“While the authors of this study have attempted to account for the family environments, it remains possible that there differences between adolescents who use marijuana and those who don’t  that explain the poor outcomes in adulthood. The other interesting finding in this paper is dose-response relationship between increasing frequency of marijuana use and increasing likelihood of poor outcomes in adulthood. Adolescents with the highest risk of poor outcomes were using daily at some point during adolescence. One question is whether there is a safe level of marijuana use;  this paper would suggest there isn’t as all groups had poorer outcomes compared to adolescents who had never used marijuana.”

Professor Max Abbott, Pro Vice-Chancellor and Dean, Faculty of Health and Environmental Sciences, Auckland University of technology, comments:

“This study, drawing together findings from three longitudinal Australian studies, provides the most robust and compelling support for the view that heavy cannabis use during early to mid-adolescence leads to some subsequent adverse mental health and developmental outcomes during later adolescence and early adulthood.  Study strengths include its large sample size, longitudinal design and adjustments made for multiple potentially confounding factors.  This means that it is likely that it was heavy cannabis use per se that led to lower educational outcomes and higher likelihood of later cannabis dependence, other illicit drug use and suicide attempts, rather than other factors associated with heavy cannabis use.  While a degree of doubt remains regarding whether or not these associations are causal, the findings are broadly consistent with those of less adequately designed previous studies.

“These study findings advance our understanding of the effects of adolescent cannabis use.  In my view they give good reason to err on the side of caution and to adopt policies that seek to prevent or delay adolescent cannabis use.  That said, this is not reason to oppose reform of cannabis legislation.   The 2012 United Nations Drug Report found that New Zealand and Australia have the highest use of cannabis in the world, around three times the international average.  The legal status quo is not effective in this country.  In my view it has failed to reduce cannabis use, needlessly criminalised tens of thousands of New Zealanders, wasted police and court resources and fuelled organised crime.  I believe the adverse health and social costs of current legislation and attempts to enforce it are likely to far outweigh any direct costs of this type stemming directly from cannabis use.

“In my view there is compelling logic supporting decriminalisation with resulting cost savings contributing to preventative and other measures that are likely to prevent and delay cannabis use.  My concern is that the study findings will be misused to support policies and law that fail to do what they are supposedly intended to and overall do more harm than good.

Could these findings apply to synthetic cannabinoids?

“It is not known whether the study findings would apply to synthetic cannabinoids.  This term covers a wide variety of substances with varying chemical composition and strengths.   In my opinion it is likely that similar results would be obtained with prolonged heavy use of some of these variants.   However, we know far less about them overall than we do about cannabis that has been studied scientifically for well over 100 years.”

Prof Thomas Lumley, Professor of Statistics, University of Auckland, comments:

“This new research confirms that correlations between cannabis use and impaired performance later in life are unlikely to be due entirely to chance or to publication bias.


“The associations in the paper are summarised by estimated odds ratios comparing non-users to those who used cannabis daily. This can easily be misleading to non-specialists in two ways. Firstly, nearly all the statistical evidence comes from the roughly 1000 participants who used cannabis less than daily, not the roughly 50 daily users – the estimates for daily users are an extrapolation.

“Secondly, odds ratios are hard to interpret.  For example, the odds ratio of 0.37 for high-school graduation could easily be misinterpreted as a 0.37 times lower rate of graduation in very heavy cannabis users. In fact, if the overall graduation rate matched the New Zealand rate of 75%, the rate in very heavy cannabis users would be 53%, and the rate in those who used cannabis more than monthly but less than weekly would be 65%.


“The research has more limited ability to conclude that the correlations are actual effects of cannabis use. It does have the advantage that cannabis use was measured before the educational and psychological variables that are supposed to be affected. In addition, some (though not all) of the variables used in attempts to adjust away other possible causes were measured before cannabis use. However, the adjustments are made at a single point in time, and are unavoidably limited in the types of information they have available.

“There are two primary causal questions in research of this sort. First, is the reduction in graduation rate or increase in depression actually caused by cannabis smoking? Second, is it a biochemical effect, or some other partly or entirely social effect? These questions are critical given the researchers’ interest in cannabis policy, where the results are relevant to the extent that they tell us what would happen to adolescents under different regulatory policies.

“An example of an explanation that fails the first criterion would be that students with (for whatever reason) more ambition or more interest in intellectual activities are less likely to become heavy cannabis users than others from a similar background, and are also more likely to do well in school.  An example failing the second criterion would be that friendships formed around the daily use of an illegal drug are less supportive of educational achievement.

“It’s not that this sort of explanation is more plausible than a biochemical effect, it’s that the analyses in are not able to distinguish one from the other: the research provides no additional evidence over and above the prior plausibility of the explanations. The data are consistent with serious biochemical effects on brain development, but are also consistent with less alarming possibilities.


“The issues here are similar to those in the research on cannabis and IQ from the Dunedin Cohort, two years ago. After that research was published, a Norwegian economist, Ole Rogeberg, argued that the analysis as performed could not rule out an explanation by socioeconomic status. It would have been reassuring to see some mention of this disagreement and its applicability to the current research, but there is none in the paper.


“In a few years there will be complementary evidence from the US states that have relaxed cannabis regulations. Combining ‘natural experiment’ data on national standardised test results in the US with the detailed cohort data from studies like the current one should then allow a more reliable assessment of the impact of cannabis.”

Our colleagues at the UK SMC also collected the following expert commentary.

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

“This is a careful study which puts together separate studies to get sufficient statistical power to investigate the effect of adolescent cannabis use on a range of outcomes. The authors conclude that daily use of cannabis before age 17 is associated with lower rates of high school completion and university qualifications, and increased risk of cannabis dependence, use of other illicit drugs, and attempted suicide.  None of the findings will surprise mental health workers, and indeed previous studies have reported similar findings for each of the outcomes separately. However, no study has examined these outcomes together or so carefully excluded possible artefacts.

“There is one important omission in the study – it does not provide data on the important question of whether the use of high potency cannabis is especially risky.

“This study reminds us that it is important to discourage cannabis use among teenagers, and that educational campaigns outlining the risks of heavy cannabis use are warranted whatever the legal status of cannabis.”

Prof David Nutt, Ex-President of the European College of Neuropsychopharmacology, said:

“This paper shows that heavy cannabis use is associated with poor social educational and mental health outcomes. This is not really surprising – it’s quite probable that the same would be true for heavy use of any intoxicating drug, perhaps even nicotine.

“The most interesting comparison would be with alcohol used in the same ways. In all cases it is likely that significant proportions of the users have pre-existing problems and seek cannabis as a way out.

“Another interesting comparison would be with the consequences of criminalisation for drug possession on the same educational and mental health outcomes – indeed was this taken into account?

“These data don’t really help with the legalisation debate as it’s been legal in Holland for decades with little impact – indeed young people may use less once it’s legal if they were rebelling by using it.”

Dr Adam Winstock, Founder of Global Drug Survey, Consultant Psychiatrist & Addiction Medicine Specialist, said:

“This is an important paper. It has combined the available research to highlight the risks cannabis use poses to young people. That the risks are greatest for those already most disadvantaged by poor mental health and educational engagement also reminds us that we should not respond to drug use in isolation of broader societal issues such as poverty and familial disintegration.

“The early onset of cannabis use is a potentially avoidable variable that compounds difficulties faced by those already most disadvantaged through socioeconomic deprivation and restricted educational achievement.

“Policies and credible messaging that delay the onset of cannabis and other drugs, including alcohol, till after 18 years old are desperately needed.”