Longitudinal studies on cannabis

Joseph Boden writes:

In 101 days, New Zealanders will determine whether or not cannabis should be legalised. In making that decision, there is plenty to learn from the and Dunedin longitudinal studies, which together have contributed a vast amount of knowledge regarding cannabis-related harm. 

The Christchurch Health and Development Study and the Dunedin Multidisciplinary Health and Development Study were both founded in the 1970s. Each includes more than 1000 participants, with individuals followed into their 40s. Along the way we have repeatedly asked about their involvement with cannabis and problems arising from this. Professor Wayne Hall of the University of Queensland, an internationally renowned figure in substance use research, has referred to our studies in the context of cannabis research as “the best designed and most informative of these [epidemiological] studies”.

These studies are the gold standard for research.

What did we find? We learned that cannabis use is very common, with as many as 80% of participants having used cannabis on at least one occasion. Rates of regular use (using at least weekly) were far lower (35%). We also found that higher levels of cannabis use, particularly at younger ages, were related to mental health problems including increased risk of cannabis use disorder and major depression. Most strikingly, earlier and heavier cannabis use was also related to increased reports of psychotic symptoms in the study, and increased risk of meeting criteria for a diagnosis of psychotic illness in the Dunedin study. Further research by both studies has found that long-term cannabis use is also associated with gum disease, impaired lung functioning, and cognitive problems.

In addition to these findings, we also found that earlier and heavier users of cannabis were less likely to complete educational qualifications, and were more likely to report being unemployed, or receiving a benefit. In addition, while only a minority of cannabis users went on to use other, possibly more dangerous illicit drugs, those participants who were heavy cannabis users in adolescence were especially at risk.

So a lot of negative outcomes associated with high levels of cannabis use. Very useful reminder that this drug is harmful.

Given our research on the risks associated with cannabis use, why do the directors of both the and Dunedin studies maintain that cannabis should be dealt with as a health issue, and not a justice issue? The reason again is related to our findings. 

First, despite being a banned substance, cannabis is commonly used across both cohorts, indicating that prohibition does not stop people using cannabis. Second, we found that those who were arrested or convicted of a cannabis offence did not reduce their use of cannabis (in fact some increased their use), suggesting that being subjected to the force of the law does not deter people from using cannabis. Third, the study found that Māori were three times more likely than non-Māori to be arrested or convicted on a cannabis offence, showing that prohibition law is enforced by the police and courts in a racially biased way. 

Collectively, our findings suggest that cannabis prohibition laws are not fit for purpose, and that in the 21st century we must deal with the problems associated with cannabis in a way that promotes health, equity and justice for all New Zealanders. The way forward is through legalisation and strict regulation as provided by the Cannabis Legalisation and Control Bill.

A useful reminder that one can agree that cannabis use is harmful but also agree that criminalising its use is not the best way to deal with it.

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