Some heavy cannabis users are up to 11 times more likely to develop schizophrenia, a drug symposium in Auckland has been told.
People with a certain gene combination that exists in 25 percent of the population had that heightened risk, Otago University's Professor Richie Poulton told the Cannabis and Health Symposium.
Risk of psychosis was elevated for those who used the drug when they were young, the Dunedin Longitudinal Study showed, but not at the levels of that gene combination.
For people who used cannabis heavily before the age of 18, the risk of schizophrenia increased by 10.3 per cent, Poulton said.
For those who used it heavily after 18 years-old, the risk increased by 4.7 per cent.
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The Dunedin study found an eight-point decline in the IQs of some early cannabis users. IQ points lost 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," Poulton said.
However, researchers cannot definitely say cannabis is the cause of schizophrenia, said Associate Professor Nadia Solowii from the University of Wollongong.
Solowii said research shows the drug 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," she said.
"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."
Cannabidiol, or CBD, is another psychoactive component of cannabis that is thought to counteract the short- and long-term negative effects of THC.
However, CBD had largely been bred out of modern cannabis due to market demand for higher THC levels, said Solowii.
This was likely to have increased the association between cannabis use and psychosis.
The Dunedin Longitudinal Study started 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 early 40s.
The Cannabis and Health Symposium runs until tomorrow, in Auckland.
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