The smoke is clearing on the link between cannabis and mental health problems, as latest research shows how the drug can hasten the onset of schizophrenia by several years.

The study which took in data from 20,000 patients with a psychotic illness has found those who smoked cannabis were diagnosed almost three years ahead of those who did not use the drug.

Dr Matthew Large, from the University of NSW’s School of Psychiatry and Prince of Wales Hospital, said the study was unique in scale 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,’ Dr Large said.

‘The results … provide strong evidence that stopping or reducing cannabis use could delay or even prevent some cases of psychosis.’

Dr Large, in a partnership with Melbourne’s St Vincent’s Hospital and the US-based George Washington University School of Medicine and Health Sciences, pooled patient data from more than 80 studies which had probed the link between psychotic illness and substance abuse.

The earlier studies had looked at the role played by cannabis, alcohol and other psychoactive substances but Dr Large’s meta-analysis was looking for an effect caused by cannabis alone.

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.

This time difference could be critical, Dr Large said, 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,’ Dr Large said.

‘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.’

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.

Dr Large said it was also estimated about one third of those Australians with a diagnosed psychotic illness also report a history of cannabis use.

Just how cannabis use could trigger a psychotic illness was not yet known, though it could hinge on genetic as well as environmental factors or be the result of ‘disrupting’ the brain during a period of ‘important neurological maturation’.

Dr Large also said it was also suspected, but not yet proven, that cannabis use made more people prone to psychotic illness.

He said those 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,’ Dr Large said.

‘We are in that process of examining epidemiological associations of cannabis and this is another piece – quite a big piece – of the jigsaw.’

The research is published in the journal Archives of General Psychiatry.

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