Cannabis use nearly doubles risk of mental illness, says study

SMOKING cannabis almost doubles the risk of psychotic mental illness such as schizophrenia, new research revealed yesterday.

The study, based on over 25 years’ data, suggested effects on the brain caused by cannabis probably explained the association with higher rates of psychosis.

It was not likely to be due to people with mental illness having a greater wish to smoke cannabis.

Researchers in New Zealand analysed information obtained on 1,055 people born in 1977 who took part in a public health investigation which followed their fortunes over 25 years.

At the ages of 18, 21 and 25, participants in the Christchurch Health and Development Study (CHDS) were questioned about their use of cannabis.

They were also assessed on various aspects of their mental health.

The team from the University of Otago used the data to work out whether high consumption of cannabis was related to increased rates of psychosis.

The scientists applied statistical models to take account of the possibility that, rather than the drug causing psychosis, mental illness encouraged a greater use of cannabis.

They also adjusted the results to account for factors such as family history, current mental disorders, and illicit substance abuse.

Prof David Fergusson, who led the study, reported yesterday in the journal Addiction, said: “Even when all factors were taken into account, there was a clear increase in rates of psychotic symptoms after the start of regular use, with daily users of cannabis having rates that were over 150% those of non-users.

“These findings add to the growing body of evidence from different sources, all of which suggest that heavy use of cannabis may lead to increased risk of psychotic symptoms and disease in susceptible individuals.”

It was estimated that daily users of cannabis had rates of psychotic symptoms between 1.6 and 1.8 times higher than those of non-users.

The scientists said the possibilities were that cannabis use was either a cause or an effect of the incidence of psychosis.

“The predominant direction of causality is likely to involve a path from cannabis use to psychotic symptoms rather than a path from psychotic symptoms to cannabis use,” the researchers wrote.