A new study has shown that cannabis use can hasten the onset of schizophrenia by several years. The study included data from 20,000 patients with a psychotic illness and found those who smoked cannabis were diagnosed almost three years ahead of those who did not use the drug. The research is published in the journal Archives of General Psychiatry.
According to Dr Matthew Large, from the University of NSW’s School of Psychiatry and Prince of Wales Hospital, this study was unique in size and it should settle debate on whether cannabis could trigger earlier mental health problems. “Results of this study are conclusive and clarify previously conflicting evidence of a relationship between cannabis use and the earlier onset of a psychotic illness… The results ... provide strong evidence that stopping or reducing cannabis use could delay or even prevent some cases of psychosis,” Dr Large said.
The patient pool was obtained by Dr Large, in partnership with Melbourne’s St Vincent's Hospital and the US-based George Washington University School of Medicine and Health Sciences, from more than 80 studies which had probed the link between psychotic illness and substance abuse. While many of these studies have also tried to evaluate the role played by cannabis, alcohol and other psychoactive substances, this meta analysis of all trials checked on cannabis alone.
Results from the studies showed that most of the patients involved had schizophrenia and, of those who were cannabis smokers, their diagnoses were seen to occur an average of 2.7 years earlier in their lives. Dr Large said this time difference could be critical as it ensured psychotic symptoms were more likely to emerge during a person’s formative years and so compounded the life-long impact. “When you see people who develop schizophrenia in their 40s and they have family around them, and an occupation, often it is a much more simple matter of prescribing some medication and providing some education ... it is not nearly as disabling…People who get it at 15 are much less likely to be able to hold down a job, to sustain relationships or complete their education,” he explained. The study also found stopping or reducing cannabis use could delay, or even prevent, some cases of psychosis.
“The risks for older people is about double, so instead of having a 1 per cent chance of developing schizophrenia you are probably likely to have about a 2 per cent chance…But for young people who smoke cannabis regularly, instead of having around a 1 per cent chance of developing schizophrenia during their life, they will end up with something like a 5 per cent chance of developing schizophrenia,” Dr. Large added.
Figures have shown that cannabis is the most widely used illicit drug in Australia with a third of the population (33.5 per cent) reporting use at some time, according to a 2007 National Drug Household Survey. From Dr. Large’s study about one third of those Australians with a diagnosed psychotic illness also report a history of cannabis use. Jan Copeland from the National Cannabis Prevention and Information Centre says campaigns need to be ramped up and better targeted. “Certainly its reputation is that it’s just a chilled-out alternative to alcohol…We know that alcohol is very bad for adolescents and we’re saying the same thing about cannabis. We always knew that there was a strong association between cannabis and developing psychotic disorders…But this gives us a very clear piece of information; that it can bring on an episode of schizophrenia up to three years earlier than would otherwise have occurred.”
However the causal relationship between cannabis and mental illness is still unclear, according to Phillip Mitchell, head of psychiatry at the University of New South Wales. “This research can’t distinguish about whether cannabis causes schizophrenia or brings it out in vulnerable people…But in many ways that’s really an esoteric academic question. The information from the study makes it very clear that cannabis is playing a significant role in psychosis…We need to take this on board to develop strategies to prevent this terrible condition occurring in young people,” he said.
Speaking of future Dr Large said cannabis smokers who developed psychotic illness early could still have done so later in life had they never used the drug, and more research was needed. “It took a long time to prove cigarette smoking caused lung cancer - it wasn’t really until 1965 that that information was firmly established…We are in that process of examining epidemiological associations of cannabis and this is another piece - quite a big piece - of the jigsaw.”
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