Arguments for legalisation imply cannabis is harmless and legalisation is beneficial. Both assumptions are incorrect.

In the last year there has been ongoing advocacy for the legalisation of cannabis in New Zealand, with a recent survey suggesting nearly two thirds of respondents think cannabis use should be legalised. These claims have been reinforced by petitions to permit the use of medicinal cannabis.

An unfortunate feature of this debate is that few contributors have discussed the harms of cannabis or the risks of legalisation.

Many contributions to this debate imply that cannabis is a relatively harmless drug and that legalisation has beneficial consequences. Both assumptions are incorrect.

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 1265 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 that:

* Cannabis use by cohort members was common, with over 75 per cent reporting use, and in the region of 15 per cent developing a pattern of heavy use and dependence at some point.

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* The use of cannabis was associated with increased risks of a number of adverse outcomes including: 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. These effects were most evident for young (under 18-year-old) users and could not be explained by social demographic and contextual factors associated with cannabis use.

Recent findings from the US

There has been a widespread assumption that the legalisation of cannabis will be beneficial by:

* Removing the adverse effects of prohibition.

* Increasing taxation to assist with health issues.

Supporters of these views have often referred to the experiences of US states in legalising cannabis, and it has widely been assumed that legalisation does not have harmful effects. This assumption has recently been challenged by two reports of the adverse consequences of medical marijuana and the legalisation of cannabis in the US.

In the first report, investigators found the legalisation of cannabis in Colorado was associated with multiple adverse consequences including: increased use of cannabis; increased risks of cannabis-related driver mortality and accidents; increased use by teenagers; and increases in hospital attendance for cannabis related outcomes.

A more recent study in JAMA Psychiatry looking at the impact of medical marihuana concluded:

"Medical marijuana laws appear to have contributed to an increased prevalence of illicit cannabis use and cannabis use disorders. State specific policy changes have also played a role. While medical marijuana may help some, cannabis-related health consequences associated with changes in state marijuana laws should receive consideration by healthcare professionals and the public."

We need a cautious approach to both the legalisation of cannabis and the introduction of medical marijuana.

What needs to be done

This recent evidence suggests that both the provision of medical marijuana and legalisation of cannabis may have adverse effects by increasing the use and harms of cannabis. At the same time it is clear the current prohibition in New Zealand has had the adverse effects of criminalising otherwise law-abiding citizens, and targeting individuals coming to official attention for other reasons, resulting in men and Maori being at increased risks of arrest and conviction for the possession of cannabis.

What is required are revisions to the current laws on cannabis which achieve the following objectives:

* Depenalisation of simple possession for individuals aged 18+.

* Increased protection of young people under 18 against the adverse effects of early cannabis use.

* Changes to the law regarding medical marijuana to permit medical practitioners to prescribe cannabis-related treatments.

* Maintenance of current laws about the supply and sale of cannabis.

* Thorough evaluation of the consequences of policy change through regular data collection on the use of cannabis and rates of cannabis-related harm.

These suggestions are intended to steer a middle course between the legalisation of cannabis and its attendant risks while at the same time avoiding the adverse consequence of prohibition.

Emeritus Professor David Fergusson is the founder of the Christchurch Health and Development Study

Disclaimer: The views stated in this commentary are my personal views on the issue of cannabis legalisation in New Zealand. They do not represent the views of the Christchurch Health and Development Study or the University of Otago.