Frequent cannabis use during adolescence and young adulthood moderately increases the risk for later psychosis, but the risk is much higher in young people with a pre-existing vulnerability to psychosis, according to an article in the January 1st issue of the British Medical Journal.

The prospective, population-based German study involved 2,437 people aged 14 to 24 years at baseline who were followed for four years: Standardized individual interviews at baseline and after follow-up were used to establish relative risk for psychosis, presence of any psychotic symptoms, and patterns of substance use.

After adjustment for influential factors such as social and economic status, childhood trauma, and use of other drugs, tobacco, and alcohol, cannabis use moderately increased the risk of psychotic symptoms. However, among people with any risk factors for psychosis (the predisposition group versus the no-predisposition group), there was a much larger, statistically significant cannabis effect that appeared to have a dose-response relation between frequency of cannabis use and development of psychosis.