Issue 39 / 20 October 2014

THE media simplify the cannabis policy debate to a choice of two options — either cannabis is harmless and so we should legalise it, or cannabis harms users so we should continue to prohibit its use.

This framing, which prevents us getting a clearer view of the adverse health effects of cannabis, has been exemplified by recent UK media coverage of my article published in Addiction.

My paper reviewed research conducted in developed countries over the past 20 years on the adverse health effects of cannabis use, particularly use starting in adolescence and continuing through young adulthood, and the psychosocial outcomes of users in their late 20s and early 30s.

The best designed and most informative research has been New Zealand birth cohort studies, which have followed up about 1000 young people, of whom a substantial proportion used cannabis during adolescence and young adulthood.

Enough of these users used cannabis often enough, and for long enough, to provide valuable information about the adverse effects of sustained daily cannabis use. Confidence in the results of these studies has been increased by similar results from studies in Australia, Germany and the Netherlands.

Cannabis does not produce fatal overdoses like heroin but occasional cannabis users can still experience adverse effects. The one of greatest public health concern is the doubling of the risk of car crashes if cannabis users drive while intoxicated. This risk increases substantially if users also consume intoxicating doses of alcohol.

Daily cannabis use over years and decades increases the risk of developing dependence on cannabis, that is, developing impaired control over cannabis use and experiencing difficulty ceasing use despite knowing that it is harming the user.

The risks of dependence are around one in 10 for anyone who uses cannabis and one in six among those who start in adolescence. This compares with one in three for smokers, one in four for heroin users and one in six for alcohol users.

Cannabis dependence is associated with increased risks of a number of adverse outcomes in young adulthood. It roughly doubles the risk of psychotic disorders (from around 1% to 2%‒3%). This is especially likely to happen if the user has a personal or family history of psychotic disorders, and if they initiate cannabis use in their mid teens.

Daily adolescent cannabis users perform more poorly at school, are more likely to leave school early, less likely to undertake post-secondary training and more likely to end up on welfare.  This pattern of cannabis use is also associated with cognitive impairment, but the mechanism and reversibility of this effect is unclear.

Daily cannabis users are also much more likely than non-users to use other illicit drugs but debate continues about why this is the case. Is it due to the characteristics of those who become daily cannabis users? Is it explained by the fact that cannabis is often supplied by the same black market as other drugs?

All these relationships between daily cannabis use and adverse psychosocial outcomes have persisted in studies that have controlled for plausible confounding variables (such as family background and childhood experiences).

Some researchers still question whether adverse effects are causally related to regular cannabis use, suggesting that they are better explained by shared risk factors.

Physical health outcomes of sustained daily cannabis use are less certain. There are fewer studies of the health effects of cannabis use that continues throughout adulthood. Daily cannabis smokers have higher rates of chronic bronchitis but evidence is conflicting on whether it impairs respiratory function in the same way as tobacco smoking.

Cannabis smoking probably increases the risks of myocardial infarction in middle-aged adults.

All this evidence indicates that daily cannabis use can harm some users. These harms are not as bad as those produced by heroin, tobacco or alcohol.

Informed public debate is needed on whether they are sufficient to justify a continuation of current policy.

Whatever policy we adopt, we should be discouraging daily cannabis use among current and future users, especially those in their mid teens. That means moving beyond the prevailing simplification of either harmless and legal, or harmful and prohibited.

Professor Wayne Hall is a Professorial Fellow and an NHMRC Fellow in addiction neuroethics at the University of Queensland Centre for Clinical Research and the Queensland Brain Institute, and visiting professor at the National Addiction Centre, Kings College London, and the University of NSW National Drug and Alcohol Research Centre.



Given the research on long-term harms from cannabis, should more resources be devoted to discouraging its use?
  • Yes - harms justify it (53%, 49 Votes)
  • No - it's no worse than alcohol (34%, 32 Votes)
  • Maybe – the harms are concerning (13%, 12 Votes)

Total Voters: 93

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5 thoughts on “Wayne Hall: Smoking out facts

  1. University of Queensland - Central Library says:

    Regarding MV crash risk, what Wayne doesn’t explain (and others) is that the doubling of risk equates to a BAC of about .02. ie compared to alcohol the crash risk for cannabis is virtually trivial.


  2. Communicable Disease Control Directorate says:

    Clearly from the information provided by Prof Hall it is adolescent cannabis use that is most likely to produce signifigant psychiatric morbidity and potentialy life long socio-economic disadvantage. This therefore is where the preventative and treatment effort must focus. Having worked in the NT and New Zealand some years ago it was apparent that the prevalence of regular cannabis use by Indigenous youth was particularly high. These are populations that can ill afford any additional disadvantage and the need to address the issue in these communities is particularly urgent.

  3. University of Queensland - Central Library says:

    Maybe adolescent use is not as bad as we thought. see:


  4. Genevieve Freer says:


    Whether cannabis is less or more harmful than tobacco or alcohol  ,there is no evidence that any of these is beneficial   in children nor teens, , so their use should not be funded by Centrelink.

    Cannabis costs money, as does alcohol,  tobacco, heroin and each of these replace healthy food and sport in poor and other  adolescents and even younger children., 

    Rather than giving Centrelink payments to parents who prefer that their children including teenagers use cannabis, alcohol, tobacco, than eat healthy food or play sport, continuing the welfare cycle introduced more than 30 years ago,  Centrelink money supposed to be spent on children should stop, and the Centrelink money should be directed to child-care centres, pre-schools, schools to supply healthy food , sport,, uniforms, books, to all children. 

    I reject the idea that parents or carers who refuse to support their children should have cash dropped in their bank accounts , for as many as twelve or more  children, referring to Centrelink payments as theri “pay”, when they have never had a job, nor intend to ever work.

    These parents or carers have children  as a cash cow for their own addictions, and introduce their addictions  to their children,, who have no other role model.

    My  opinion is that only those who use Cannabis promote Cannabis use ,  with  a delusion of entitlement, similar to the alcoholic who thinks that someone owes them a drink, and the smoker who thinks that they have the right to smoke in the car with children while we pay for their tobacco.

  5. University of Queensland - Central Library says:

    Thank you Wayne Hall for this important information. Given the increasing transgenerational use of cannabis in many families, it would be logical to fund research into the teratogenic effects. Inheriting a risk of addiction is well known through grounded theory, but what is the effect on babies in utero, during breastfeeding and having cannabis blown in the face as a ‘calming’ mechanism? Please can we raise our voices for the unborn in this debate?

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