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Chronic cannabis use in early adolescence can make some people up to 11 times more likely to develop schizophrenia, the New Zealand Drug Foundation's Cannabis and Health Symposium in Auckland has been told.

Professor Richie Poulton, Director of the Dunedin Multidisciplinary Health and Development Research Unit, said findings from the Dunedin Longitudinal Study showed an elevated risk of developing psychosis in the presence of early cannabis use.

"For people that use cannabis heavily before the age of 18 the risk of schizophrenia increases by 10.3 percent. For those who use heavily after 18 the risk increases by 4.7%."

However, he said that for some people with a certain gene combination the risk is much, much worse (around eleven-fold) and that this gene combination exists in a quarter of the population.

The Dunedin Longitudinal Study commenced 40 years ago and closely follows the lives and health of 1037 babies born in 1972-1973, 981 of whom are still involved and are now in their late 30s. It considers a very wide range of health effects so is very aware of what might normally be confounding research factors, Professor Poulton said, so its findings are well regarded internationally.

The study has also found an 8 point decline in the IQs of some early cannabis users and these IQ points were not fully recovered when cannabis use ceased.

"What this all suggests is that adolescence is a very sensitive period in brain development and policy makers need to find ways of delaying cannabis use as much as possible for young people," Professor Poulton said.

Associate Professor Nadia Solowij, from the University of Wollongong’s School of Psychology, said that while we can’t definitely say cannabis causes schizophrenia, research shows it is what is known as a 'component cause' in that it can trigger psychosis in vulnerable people.

"Cannabis receptors are abundant in the human brain and are associated with higher functions such as attention, memory, learning and planning as well as pain, appetite and sleep. Bombarding the brain with THC, the main psychoactive ingredient in cannabis, can interrupt the natural balance of cannabinoids in the brain and can produce very similar kinds of impairments to those suffering from schizophrenia, even in people not already prone to the disorder," Ms Solowij said.

Cannabidiol, or CBD, is another psychoactive component of cannabis that is thought to counteract the short- and long-term negative effects of THC. But Ms Solowij said it was concerning that CBD had largely been bred out of modern cannabis due to market demand for higher THC levels. This is likely to have increased the association between cannabis use and psychosis.

The Cannabis and Health Symposium runs from 26-29 November and seeks to broaden New Zealand’s discussion of issues around cannabis such as recent research about its effects, whether there is a need for cannabis law reform and the best ways of addressing cannabis-related harm.

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