Research has suggested that people who use cannabis are more likely to develop psychotic illnesses at an earlier age.
Published in the Archives of General Psychiatry, the study conducted a meta-analysis of previously published studies.
The team from the University of New South Wales and Prince of Wales Hospital in Australia found that individuals they assessed who used cannabis developed psychosis about 2.7 years younger than those who did not use the drug.
Those who used any type of substance developed psychosis about two years younger. However, the use of alcohol only was not associated with the age at onset of psychosis.
"The results of this study provide strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis," the authors concluded.
They suggested that reducing the use of cannabis could be "one of the few ways of altering the outcome of the illness because earlier onset of schizophrenia is associated with a worse prognosis and because other factors associated with age at onset, such as family history and sex, cannot be changed".