The findings highlight the need for reducing marijuana use, particularly considering the substantial increase among young people in recent years, said Joseph M. Rey, a professor of child and adolescent psychiatry at the University of Sydney who wrote an accompanying editorial to the studies that appear in the Nov. 23 issue of the British Medical Journal.
The connection between schizophrenia and depression and marijuana use has been seen in previous studies and this adds further confirmation of a link, Rey told United Press International. "These findings strengthen the argument for cannabis use increasing the risk of schizophrenia and depression and provide little support for the belief that" people already prone to mental illness are using the drug as a way of self-medicating, he said.
But, he noted, "Whether cannabis use triggers the onset of schizophrenia or depression in otherwise vulnerable individuals or whether it actually causes these conditions in non-predisposed people is not yet resolved."
Marijuana advocates agreed with Rey the studies did not show a definitive link between use of the drug and mental disorders. "This really does require some further study," Paul Armentano, spokesperson for the National Organization for the Reform of Marijuana Laws, told UPI. "We really can't draw any hard and fast conclusions based on the evidence that's out there," he said.
Armentano pointed out overall marijuana might be much safer than many commonly used legal drugs. "When you compare marijuana's overall health risks they really seem to be lower than legal drugs such as tobacco and alcohol," he said. He noted NORML does not advocate marijuana use for adolescents and believes the drug should only be used by responsible adults for recreational or medical purposes.
In one study, Stanley Zammit of the University of Wales in Cardiff and colleagues reanalyzed a Swedish study done in 1970 because although the research originally found an association between marijuana use and schizophrenia, there was some concern that the mental disorder could already have been present before the drug use started or it could have been due to other drugs.
The reanalysis found that marijuana -- and not other drugs -- most likely accounts for the increased risk of schizophrenia in the more than 50,000 young men who were studied. "Men who had used cannabis by age 18-20 had an increased risk of developing schizophrenia over the next 27 years," Zammit told UPI. "This increase was dose-dependent, in other words the more they had used cannabis, the greater the risk," he said.
For example, men who had used it more than 50 times had a 300 percent increase in risk compared to non-users. The risk dropped to a 40 percent increase in those who had used it less than 10 times.
Other factors such as personality traits or use of other drugs did not account for the association between schizophrenia and marijuana usage, Zammit said. They also found that marijuana use began before the onset of schizophrenia. "Nevertheless we cannot be certain that the increased risk is due to cannabis -- there could be other explanatory factors we haven't considered or were unable to measure," he said. "Overall though, the most likely explanation for the increased risk is that of cannabis use."
The second study of schizophrenia confirmed marijuana use increased the risk of schizophrenia later in life in a group of New Zealand adolescents who ranged in age from 15 to 18. Those who began using marijuana at 15 had the greatest risk of developing schizophrenia as adults. Cannabis use preceded the onset of the mental disorder in this study also.
"Although most young people use cannabis in adolescence without harm, a vulnerable minority experience harmful outcomes," the researchers, Louise Arseneault of King's College in London, UK, and colleagues, write in the journal. This suggests "policy and law makers should concentrate on delaying onset of cannabis use," they add.
In the study showing a link to depression, 1,600 students from 44 secondary schools in Australia were monitored for seven years. Frequent use of marijuana correlated with later depression and anxiety in teenage girls. Daily users had a more than five-fold increase in the risk of developing depression and anxiety later in life. "Measures to reduce frequent and heavy recreational use seem warranted," George Patton, a professor of adolescent health at the Murdoch Children's Research Institute in Victoria, Australia, and colleagues, write in the journal.
(Reported by Steve Mitchell, UPI Medical Correspondent, in Washington)