Professor David Fergusson and colleagues at the School, who have been gathering data on a cohort of 1000 people for 25 years, have just published a research paper which suggests that heavy users of cannabis are more likely to suffer from psychotic symptoms.
The new research results are published in the international journal Addiction, and support growing evidence that cannabis can damage mental health.
Professor Fergusson says his research, based on participants in the Christchurch Health and Development Study, indicates that there is a clear increase in psychotic symptoms after the start of regular use, with daily users of cannabis having symptom rates that are 1.5 times more than non-users.
"These findings add to a growing body of evidence from different sources which suggest that heavy use of cannabis may lead to increased risk of psychotic symptoms and illness in susceptible individuals," he says.
Professor Fergusson says one of the problems with this area of research in the past has been that it is difficult to determine the extent to which cannabis influences psychotic symptoms or whether people with these symptoms tend to use cannabis. However the researchers were able, through statistical models, to adjust for the fact that psychosis encouraged cannabis use, and for other factors associated with cannabis.