18th January 2019, Volume 132 Number 1488

Joseph M Boden, David M Fergusson

It appears likely that New Zealanders will have a chance to participate in a referendum on possible changes to the law concerning recreational cannabis before the next election in 2020.1 This has occurred in the context of ongoing advocacy for the legalisation of recreational cannabis use in New Zealand, with a recent survey suggesting that nearly two thirds of respondents are of the view that cannabis use should be either decriminalised or legalised.2 In addition, the New Zealand Parliament has very recently passed a Medicinal Cannabis bill, and there has been a strong recommendation by the recent New Zealand Mental Health and Addictions Inquiry for the decriminalisation of drug use in general.3

An unfortunate feature of the debates on changes to cannabis law is that relatively few contributors have discussed either the harms of cannabis or potential risks of legalisation. Most contributions imply that cannabis is a relatively harmless drug, and that cannabis law change will only have beneficial consequences. We would argue that, on the basis of evidence generated by longitudinal studies based in New Zealand, both assumptions are incorrect.

New Zealand-based research on outcomes associated with cannabis use

New Zealand has some of the richest data on the adverse consequences of cannabis use coming from two major studies: the Christchurch Health and Development Study (CHDS) and the Dunedin Multidisciplinary Health and Development Study (DMHDS). The CHDS is a study of a cohort of 1,265 children born in 1977 who have been studied to the age of 35. The study has now published 30 scientific papers on the issue of cannabis. This research shows resoundingly that cannabis use by cohort members was common, with over 75% reporting use, and in the region of 15% developing a pattern of heavy use and dependence at some point.4

The CHDS has also provided a wealth of cutting-edge research of the adverse health and psychosocial outcomes associated with cannabis use. Data from the CHDS has shown that cannabis use is associated with: educational delay; welfare dependence; increased risks of psychotic symptoms; major depression; increased risks of motor vehicle accidents; increased risks of tobacco use; increased risks of other illicit drug use; and respiratory impairment.5 These effects were most evident for young (<18 years) users, and in particular heavier users of cannabis, and could not be explained by social demographic and contextual factors associated with cannabis use. This research clearly establishes the facts that: a) cannabis use is not harmless; b) it has its greatest impacts on young users who are vulnerable to the neuro-psychological effects of cannabis.5

At the same time, evidence from the CHDS suggests that the prohibition of cannabis is also a cause of some harm. In an analysis of the CHDS cohort following the age 21 assessment, Fergusson et al6 showed that males and Māori cohort members reported disproportionately higher rates of arrest and conviction for cannabis-related offences. These findings indicated that the laws prohibiting cannabis were being applied in a biased manner. Furthermore, the analysis showed that cannabis use did not decrease following arrest/conviction, suggesting that prohibition generally failed to reduce cannabis use in the cohort.

Some of the literature and advocacy for legalisation of cannabis has attempted to show that cannabis is no more harmful, or even less harmful, than alcohol. As Hall7 points out however, it is clear that: a) the harms of cannabis and alcohol cover quite different domains of functioning; and b) because cannabis is an illegal drug, the harms associated with it may have been underestimated, as use has been suppressed to some degree by its legal status.

Benefits and risks of legalisation

There are varying views in the literature on the adverse effects of the legalisation of cannabis with many commentators expressing the view that legalisation is unlikely to have adverse effects, and may in fact have positive effects, including: reducing income for criminal cartels, increasing tax revenues for the state; and avoiding criminalising large numbers of users of the drug. In addition, it could be argued that the legalisation of cannabis could lead to increased funding for cannabis-related health and psychosocial problems (including cannabis dependence), funded via increased tax revenue. It should be noted that, in US states where cannabis has been legalised (such as Colorado), increased tax revenues have been reported.

The assumption that cannabis legalisation will have primarily positive effects has recently been challenged by an evaluation of the consequences of cannabis legalisation. In a recent review of the literature, Hasin8 reported that changes in both medical and recreational cannabis laws in the US have thus far resulted in mixed effects. Hasin found that there was little evidence that cannabis use among adolescents had increased, but this was not the case for adults, among whom both cannabis use and cannabis use disorders increased. Furthermore, there was evidence of increases in cannabis-related emergency department visits, driving under the influence of cannabis, and accidental exposure to cannabis in children. This review makes it clear that cannabis legalisation can increase both the use of cannabis and cannabis-related harms.

A cautious way forward

The facts above are in sharp contrast to the current public debate about cannabis, which has failed to discuss extensive local evidence on the harms of cannabis and the negative consequences. At the same time it is clear that the current prohibition of cannabis use has adverse consequences of both criminalising otherwise law abiding citizens, and being used to apply cannabis laws to individuals coming to official attention with this resulting in males and Māori being at increased risks of arrest and conviction for the possession of cannabis.6

It is clear that any changes to the legal status of cannabis should be made with caution, and should not follow the model currently used to deal with alcohol in most Western countries. Hall7 has argued that, because of the likelihood that trade in legalised cannabis will be controlled by large business conglomerates, there is little reason to expect that legalised cannabis will be more heavily regulated than alcohol, which has been the subject of increasingly liberalised regulation over the past 50 years. Nonetheless, we would argue that it is critical for any change in the legal status of cannabis to be undertaken with caution, and to be fully evaluated at each stage to determine the extent to which these changes are leading to increased cannabis-related harm.

Given these considerations, what we would propose is the development of laws and policies that both discourage the use of cannabis and that also avoid criminalising recreational users of the drug. This proposal represents a progressive process of policy development which begins with the depenalisation of cannabis possession, the increased protection of young people and the treatment of cannabis-related harm as a health issue. The key elements of this policy are:

  1. Simple possession of cannabis by those over 18 would be decriminalised, as would supply of small amounts to adults, as recommended by the recent Mental Health Inquiry.3
  2. Penalties for the supply of cannabis to those under 18 would be increased.
  3. Investments in mental health services for those with cannabis use disorder and cannabis-related conditions would be increased, again in line with the recent Mental Health Inquiry.3

The general aim of this policy is to an attempt to steer a middle course between the shortcomings of strict prohibition and the risks of legalisation, and represents the first step in a longer-term strategy to address the issues raised by cannabis. Specifically we would propose that the first-stage decriminalisation process is evaluated at regular intervals by assessing the prevalence of cannabis use and cannabis-related harms. If this evaluation shows that that the decriminalisation process reduces harms, the next stage would be to move towards further liberalisation of cannabis laws. An advantage of this approach is that by the time the evaluation of decriminalisation is complete, clearer evidence on the costs and benefits of cannabis legalisation in the US and elsewhere will be available.

The issue of cannabis legalisation has been a highly emotive area with strong opinions often being expressed. Among these have been that cannabis is a low-risk drug which is less harmful than alcohol and that the legalisation of cannabis is beneficial and does not have harmful consequences. Neither of these claims withstands critical inspection; cannabis has multiple harmful effects which are particularly evident for young users, and the extent to which legalisation is beneficial is by no means clear. Given this context, the most prudent course of action for New Zealand to follow is to develop policies which eliminate the adverse effects of prohibition while at the same time avoiding the possible adverse consequences of full legalisation.


At the present time there are continuing debates on the legal status of cannabis in New Zealand. Many of these debates have not given sufficient consideration to evidence concerning cannabis-related harm, much of which has been gathered here in New Zealand by the Christchurch Health and Development Study (CHDS) and the Dunedin Multidisciplinary Health and Development Study (DMHDS). We present a summary of this evidence, and recommendations for a cautious path forward for changing cannabis laws in New Zealand that aims at reducing cannabis-related harm.

Author Information

Joseph M Boden, PhD, Research Associate Professor, Department of Psychological Medicine, University of Otago, Christchurch; David M Fergusson (deceased), Emeritus Professor, Department of Psychological Medicine, University of Otago, Christchurch. 


Joseph M Boden, PhD, Department of Psychological Medicine, University of Otago, Christchurch, PO Box 4345, Christchurch 8140.

Correspondence Email


Competing Interests



  1. Collins B. NZ may vote on marijuana legalisation in 2019 Wellington: Radio New Zealand; 2018 [Available from: http://www.radionz.co.nz/news/political/357982/nz-may-vote-on-marijuana-legalisation-in-2019
  2. New Zealand Drug Foundation. New survey results show legal cannabis a real possibility Wellington: New Zealand Drug Foundation; 2018 [Available from: http://www.drugfoundation.org.nz/news-media-and-events/new-survey-results-show-legal-cannabis-a-real-possibility/
  3. Mental Health and Addiction Inquiry. He Ara Oranga: report of the Government Inquiry into Mental Health and Addiction. Wellington: Mental Health and Addiction Inquiry; 2018.
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