Smoking cannabis as a teen increases risk of depression and suicide as a young adult, says study

Smoking cannabis as a teenager increases the risk of depression and suicide during young adulthood, according to a new study.

Individual risk remains moderate to low, but because so many teens are smoking cannabis, there is potential for large numbers of young people to be affected, according to findings published in the JAMA Psychiatry journal on Thursday.

However the researchers, led by Gabriella Gobbi from McGill University, Canada, didn’t find a link between marijuana use and increased risk of anxiety.

The team said their findings highlight the importance of efforts aimed at educating teenagers about the risks of using marijuana. “This is an important public health problem and concern, which should be properly addressed by health care policy,” they wrote.

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Cannabis is the world’s most widely used illicit drug, with 3.8% of the global population having used cannabis in the past year.

Marijuana is commonly used by many teenagers worldwide, but not much has previously known about how that use might impact mood and risk of suicide later in life.

For this review, the scientists analysed the combined the results of 11 studies with about 23,300 people and found marijuana use during adolescence before age 18 was associated with increased risk of depression and suicidal thoughts or attempts during young adulthood between the ages of 18 and 32.


Marijuana use during adolescence was associated with increased risk of depression and suicidal thoughts or attempts during young adulthood, the study said.

Dr Joe Boden, deputy director of the Christchurch Health and Development Study at the University of Otago, said the review both confirms and reinforces previous findings on the adverse psychosocial effects of regular cannabis use by teenagers aged between 15 and 17.

“This group of cannabis users represents approximately 5 to 10 per cent of the adolescent population, with these individuals being at significantly increased risk of co-occurring mental health and substance use problems, as well as engaging in anti-social behaviour,” he said.

“Furthermore, individuals in this group will also display higher levels of risk-taking in general, and are more likely to leave school early.”

Boden said the research should feed into New Zealand’s current debate around legalisation of cannabis.

“The findings of this study further reinforce our concerns about the public health implications of any changes we may choose to make to cannabis laws in New Zealand,” he said. “Prior studies have shown that cannabis prohibition has not impeded young people from obtaining and using cannabis.

“The study by Gobbi and colleagues emphasises the fact that adolescents are a particularly vulnerable group in terms of cannabis consumption, and any changes we make to the laws need to be both: a) designed in such a way as to keep cannabis out of the hands of adolescents; and b) thoroughly evaluated after implementation to ensure that we are not increasing the amount of cannabis-related harm in our society.”


The Government has confirmed a cannabis referendum will be held at 2020 election. Christchurch residents offer their views.

However some experts sounded a note of caution around the findings. Dr Lindsey Hines of the University of Bristol in the UK said that while the study represented “a clear, good quality estimate of the association between using cannabis as a teenager and mental health in adulthood”, it didn’t fully prove causation.

“We know that cannabis use co-occurs with anxiety, depression and self-harm in teenagers, but this research suggests that teenage cannabis use is still related to mental health in later years. However, we don’t know if cannabis use as a teenage is causing these adult mental health problems. It could be that these behaviours are all due to shared underlying risk factors, such as early adversity or genetics,” said Hines.

“It’s also important to note that these results don’t tell us if the effects are specifically due to cannabis use during teenage years. It may be that people who were smoking cannabis as a teenager have carried on smoking cannabis as adults, which may explain some of the relationship to mental health.

“This research highlights the vital work we still need to do to understand if, and how, smoking cannabis as a teenager can have effects later in life.

“Amongst teenagers using cannabis the effect of use on education and social relationships, the frequency or heaviness of cannabis use, and the strength of the drug, are all likely to relate to differences in lifetime mental health outcomes.”