NZ will vote on legalising marijuana on September 19. Image: Getty

What 40 years following thousands of NZ people tells us about cannabis harm

Longitudinal studies have a huge amount to teach us about the impacts of using cannabis, writes Joseph M Boden, director of the Christchurch Health and Development Study.

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 Christchurch 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”.

Why are longitudinal studies so important in understanding cannabis-related harm (or indeed, many behaviours and life circumstances)? First, in longitudinal studies, we observe people on repeated occasions. Repeating observations allows us to capture events which might occur over a long period, or that have a particular pattern or trajectory. This generally leads to more precise estimates of behaviour such as cannabis use. Second, longitudinal studies (including ours) are often carried out prospectively. This means that assessments are done at or near the time that the behaviour occurs, which increases accuracy, due to the fallibility of human memory for past events. 

Third, longitudinal studies, particularly those such as ours in which participants were enrolled at birth, have the ability to collect data on a wide range of family, personal, and social circumstances which can influence the relationship between an event (for example, using cannabis before the age of 15) and an outcome (such as failing to attain NCEA levels). By accounting for these factors, we can use correlational data to model a causal relationship. In this example, the question is “What is the relationship between using cannabis prior to age 15 and not completing secondary qualifications?”, and part of the answer lies in ruling out the influence of factors and circumstances other than early cannabis use that contributes to this relationship.

The Christchurch and Dunedin studies have additional notable features. One is that both are representative of the population of each city at the time the study was established. For example, in the Christchurch study, 97% of the infants born between April and August 1977 were enrolled in the study. This means that people from all walks of life were included, and therefore when we estimate the prevalence of a behaviour such as cannabis use, it is more likely that these estimates will be accurate. A second key feature is that over time, both studies have had excellent retention of their cohorts. In longitudinal studies it is not possible to replace participants, and people often drop out of studies in predictable ways. For example, in both studies, those born into families of lower socioeconomic status were more likely to drop out over the years, and as larger numbers of people drop out, the more likely it is that the data will be biased. Thankfully, for both of our studies this has not been a cause for concern.

Research into cannabis use began in both studies when the cohort members were in early adolescence (1980s and 1990s). The period when this research was being conducted was a time at which youth cannabis use levels were quite high in New Zealand. At mid-adolescence we began assessing both their frequency of cannabis use, and also whether they were experiencing any problems related to cannabis, in order to establish whether cohort members met criteria for a cannabis use disorder. Using these data, we were able to estimate how many cohort members had used cannabis, assess rates of disorder, and link cannabis use to other health, mental health, and social, economic and criminal justice outcomes. 

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 Christchurch 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.

Given our research on the risks associated with cannabis use, why do the directors of both the Christchurch 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 Christchurch 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.

Professor Joseph M Boden is director of the Christchurch Health and Development Study at the University of Otago, Christchurch

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