A new Australian study has found conclusive evidence that using cannabis can hasten the onset of psychotic illnesses such as schizophrenia.

Researchers reviewed more than 20,000 patients and found cannabis users experienced psychosis more than two-and-a-half years earlier than non-cannabis users.

The study also found stopping or reducing cannabis use could delay, or even prevent, some cases of psychosis.

The risks are especially high for young people whose brains are still developing.

Chief investigator of the study Dr Matthew Large, who is from the University of NSW and the Prince of Wales Hospital, says the results reinforce the need for warnings about the harmful effects of cannabis.

"What our research has found is that in addition to early cannabis smoking bringing on schizophrenia, it brings schizophrenia on [earlier] by an average of 2.7 years," he said.

"That's earlier than you would have otherwise developed it had you not been a cannabis smoker."

He says the earlier the smoking starts and the more frequent the use, the greater the risks.

"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," he said.

"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."

But 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," he said.

"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."

With around 33 per cent of the Australian population and 18 per cent of secondary school students using the drug, 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," she said.

"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."