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Cannabis: What's the harm?

Christchurch campus

Friday, 25 May 2018

A researcher of the effects of cannabis says any law change to free up the drug’s availability needs to take account of scientifically-robust data showing regular use in young people is associated with a higher risk of mental health issues, use of other substances, and lower levels of achievement.

The Labour Party agreed to hold a public referendum on legalising cannabis for personal use at or before the 2020 general election as part of its confidence and supply agreement with the Greens. The Government now says a referendum could be held ahead of the next election to ensure it did not overshadow the election campaign.

Associate Professor Joe Boden of the University of Otago’s Christchurch Health and Development Study (CHDS) outlined his study’s more than 20 years of data on cannabis use and its effects at a Health Precinct public talk. He says CHDS findings shows more than 80 per cent of middle-aged Kiwis have used cannabis at least once with little long-tern harm. But crucially, in young, regular users, the drug can later effect their mental health, use of other illegal drugs and earning potential, suggesting any law change should restrict access and protect this vulnerable group.

The CDHS is a world-renowned longitudinal study whose researchers have followed more than 1,000 Cantabrians born in Christchurch in 1977, including their substance use.

Associate Professor Boden crunched the numbers relating to cannabis use and harm for the public talk, and found:

  • More than 80 per cent of CHDS participants, now aged in their early 40s, had used cannabis at least once in their lives.
  • For the majority of people, casual or infrequent use of the drug is not associated with long-term negative outcomes.
  • Those who used the drug at least weekly during their teenage years were almost twice as likely as others to experience symptoms of psychosis than infrequent or non-users.
  • Those who used the drug at least weekly up to age 25 were over 10 times more likely to use other illegal drugs.
  • Less than 20 per cent of those who started using cannabis before age 15 achieved a tertiary qualification, compared with nearly 30 per cent among those who did not use cannabis before 18.
  • By age 25, those who used cannabis at least weekly as a teenager were three times more likely to experience long-term unemployment than those who did not use cannabis, or who used cannabis very infrequently.


Associate Professor Boden says the public should be aware of scientific evidence around cannabis before the proposed referendum on legalising cannabis.

“The CHDS data, along with data from other New Zealand and international studies, suggests that the harms of cannabis are most pronounced for those who begin using at younger ages, or who use cannabis heavily during adolescence. Any change to the law concerning cannabis needs to be undertaken in a way to reduce harm in this group, and in particular to provide resources for the treatment of those who develop cannabis dependence.”

Boden says the evidence from his research group over more than 20 years suggests it is important any law change protects the most vulnerable and is not just a change to the ‘open slather free market’ model alcohol currently enjoys, with the many known associated harms. The impact of any law changes needs to be carefully studied, he says.

For further information, contact:

Associate Professor Boden
Tel +64 3 372 6744

Kim Thomas
University of Otago, Christchurch
Senior Communications Adviser 
Mob +64 27 222 6016

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