Cannabis is one of the most widely available illicit drugs in New Zealand - but what impact is it having on our wellbeing?
Research from the 2015 New Zealand Health Survey shows that 11 per cent of people aged over 15 have used cannabis in the past 12 months, with one third of this group using it at least weekly. The survey shows it is most widely used by people aged 15 to 24, with 23 per cent of this group having used it in the past year - but when it comes to regularity of use, the over-55s are the most prolific users.
Although difficult to overdose, cannabis can be physically and psychologically addictive. It can affect road safety - one third of New Zealand cannabis users have admitted to driving under the influence of cannabis, and in the US state of Washington the AAA reports fatal car accidents involving drivers who recently used marijuana has more than doubled since legalisation.
The debate around legalisation is a constant feature in our media, and research around the effects of long-term cannabis consumption in New Zealand is an important consideration. This week, data published in the journal JAMA Psychiatry added to a previous PNAS article to give us a better understanding of how cannabis may be affecting our nation.
The study looked at 1037 individuals born in Dunedin in 1972-73 and has followed them to age 38 through the famous Dunedin Multidisciplinary Health and Development Study. Controlling for tobacco use, childhood health and childhood socioeconomic status, they tested whether cannabis use from ages 18 to 38 was associated with mental or physical health at age 38, after assessing the frequency of cannabis use and their health at 18, 21, 26, 32, and 38.
This remarkable study relies on confidentiality, allowing participants to be honest about their use of illegal drugs without fear of reporting to the authorities. It is therefore probably the most accurate long-term cannabis study in the world.
When it came to physical health, the results were surprising, with marijuana consumption having no negative impact on a dozen health factors including lung function, systemic inflammation, BMI, or metabolic health. The only significant adverse impact was on periodontal health with cannabis use being associated with tooth loss. This is very different to results from the tobacco smokers in the same cohort who showed worse lung function, systemic inflammation, and worse metabolic health at age 38. The theory is a heavy smoker can smoke over 20 cigarettes a day, but heavy cannabis users will still only smoke 2-3 joints a day, meaning their overall smoke exposure is much lower.
The news is not so good when it comes to mental health. The research showed that teens who started smoking marijuana before 18 and who were diagnosed as being addicted to cannabis by 38 experienced a significant drop in IQ in early adulthood. The IQ drop was not seen for users who began smoking after age 18, implying that cannabis consumption can have a permanent negative effect on teenagers' developing brains.
Wherever the debate over the legalisation of cannabis takes us, scientists and policy makers will need to continue gathering evidence.
This study is just one of many showing the powerful information that the Dunedin Multidisciplinary Health and Development Study has in helping understand health and diseases.