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

An Australian study has revealed that the briefest use of cannabis can physically change teen's brains.

They grow bigger - extra grey matter in places that typically have lots of cannabinoid receptors.

But this isn't good news. It suggests that even teens who have only smoked once or twice can be physically affected by using cannabis, rather than it being harmless.

123RF A new study shows teens who smoke dope even briefly can change their brains and become more anxious.

And the study also links that extra grey matter change to increased anxiety.

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​The study was led by Catherine Orr, of Melbourne's Swinburne University of Technology. It looked at data revealed from a large research programme into adolescent brain development and mental health that looked at teens from Ireland, England, France and Germany.

ROBERT CHARLES/STUFF About 13 per cent of New Zealand secondary school students have tried cannabis, says the NZ Drug Foundation.

Orr focused on a small subset of this group - 14-year-olds who had only used cannabis once or twice. There were only 46 in the study.

Her team found brief cannabis use in these teens caused areas of the brain to grow extra grey matter and they could rule out other factors like nicotine, gender, alcohol and socioeconomic status.

They also found a link between the extra grey matter in some sites and increased anxiety.

DAVID WALKER/STUFF New Zealand is to hold a referendum on cannabis for personal use in 2020.

Orr says while they can't tell how how cannabis causes the growth of extra grey matter, tests in animals suggest cannabis might interrupt "neural pruning" which is a typical part of adolescent brain development.

"How concerning this is remains to be seen. We do not know whether these effects persist into adulthood and how they interact with other activities teenagers engage in that may be reflected in brain structure, such as playing a musical instrument, learning a language, drinking alcohol or playing video games."

Orr says while the link between the effects and higher anxiety at age 16 was true for the sample, it is a small sample and more research is needed to show the wider risk.

"The most important finding from the study is that there was a range of effects in the brains of the teens who had used cannabis."

On average, the brain grey matter size grew just over 5 per cent, but some teens showed a more than 20 per cent increase.

"More research is needed so we can understand why some people may be more vulnerable to brain effects of cannabis at even the earliest stages of use. It might give insight into why some people transition to substance misuse while others do not.

"Also, if we can identify some of the factors that place people at greater risk of these brain effects, we need to let people know what they are so that they can make informed decisions about their substance use."

The research is published in JNeurosci.

New Zealand will hold a referendum on the legality of personal cannabis use at the 2020 general election.

The New Zealand Drug Foundation says about 13 per cent of secondary school students say they use cannabis, but only 3 per cent are likely to use it weekly.