Boys genetically predisposed to schizophrenia who try cannabis before 16 experience changes in brain development according to a new study, in a finding that could help researchers understand why some people are more vulnerable to the effects of the drug.
The study, published Wednesday in the journal JAMA Psychiatry, found that the boys developed a thinner cerebral cortex than their peers over time, which could signal that cannabis somehow interferes with the development of that part of the brain. The heaviest cannabis users experienced the most significant changes.
"We have a structural difference in the brain that appears to be there only if you combine cannabis use before 16 and a genetic risk for schizophrenia," said Tomas Paus, lead author of the study and the Tanenbaum chair in population neuroscience at Toronto's Baycrest Centre for Geriatric Care.
Neither boys with a low genetic schizophrenia risk nor girls experienced the same brain changes, the authors noted. To conduct the study, researchers looked at the brain scans, cannabis history and genetic schizophrenia risk factors of nearly 1,600 adolescents in Quebec, Britain, Germany, France and Ireland. Researchers looked at the brain scans of participants several years after cannabis use to look for brain changes. To determine a person's schizophrenia risk, researchers used a tool that looks at more than 100 different genetic locations to find changes associated with the disease.
Dr. Paus highlighted the fact they have not proven a cause-and-effect relationship between cannabis and brain changes in males with a genetic schizophrenia risk. It's also unclear whether the observed changes are a precursor to mental illness.
But the findings are adding to the body of evidence suggesting a link between cannabis use and schizophrenia risk, particularly among those who are predisposed to the disease, according to Romina Mizrahi, director of the Focus on Youth Psychosis Prevention and research program at Toronto's Centre for Addiction and Mental Health.
Dr. Mizrahi, who was not involved with the study, said the findings can help researchers understand why some people are vulnerable to the effects of cannabis while others are not.
"This study is getting us closer to that," she said.
Previous research has come up with conflicting answers about the relationship between cannabis and schizophrenia. One of the things that makes this new study unique, Dr. Mizrahi said, is that researchers carefully assessed participants' genetic schizophrenia risk factors. Doing so allowed researchers to zero in on high-risk individuals and spot patterns among cannabis users that might not have appeared if all study participants were looked at as a group.
Dr. Paus said it's possible that adolescents with a family history of mental illness who use cannabis may "put extra stress on the developing brain."
According to a 2013 UNICEF report, Canadian youth are the top cannabis users when compared to their peers in other developed countries. About 20 per cent of young Canadians report using cannabis at least once in the previous year, according to the Canadian Alcohol and Drug Use Monitoring Survey. Dr. Mizrahi said Canada must do more to educate adolescents about the risks of cannabis, particularly among those who may be most vulnerable to the effects.
One of the main messages is for young people to delay trying cannabis at least until they are older than 16, she said.
Last year, CAMH called for policymakers to legalize cannabis in Canada and introduce strict controls as a way to reduce the high rate of use among young people. Dr. Mizrahi said under such a system, marijuana sales would be more tightly controlled than tobacco. There should also be a broad campaign to educate youth about the potential risks, she said.
Cannabis – and whether to regulate it – has become a significant issue in the federal election campaign. Liberal leader Justin Trudeau has said he would move to legalize and restrict the sale of marijuana as a way of combatting high rates of use among adolescents, and NDP leader Thomas Mulcair has said the party will decriminalize marijuana for personal use.
Conservative leader Stephen Harper has said he opposes legalization of marijuana and would invest more money in police. The party has also run attack ads criticizing Mr. Trudeau for his stance on legalization.