The extent and nature of the link between cannabis use and psychosis is one of the most controversial areas of psychiatry. A team of researchers, led by Matthew Large from the Prince of Wales Hospital in New South Wales reviewed 83 studies into the relationship between cannabis use and the age at which people become ill with psychosis. They found that people who used cannabis became ill, on average, 2.7 years earlier than those who didn't with people who used any illegal drugs becoming ill 2 years earlier. Alcohol use was not associated with an earlier onset of psychosis.
Large, Matthew - Cannabis use and earlier onset of psychosis Archives of General Psychiatry