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, the researchers found.

Professor David Fergusson and colleagues at the School, who have been gathering data on a cohort of 1,000 people for 25 years, have just published a research paper that suggests heavy users of cannabis are more likely to suffer from psychotic symptoms. ("Cannabis" includes any of several mildly euphoriant, intoxicating hallucinogenic drugs, such as marijuana or hashish, prepared from various parts of the hemp plant.)

Growing Body of Evidence

The new research results, which are published in the international journal Addiction, support growing evidence that cannabis can damage mental health.

Based on the experience of participants in the Christchurch Health and Development Study, the research 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, Professor Fergusson says.

"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.

Chicken, Egg Question

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 versus 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.