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.
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.