NEW YORK (Reuters Health) - Researchers from Spain have found a strong and independent link between cannabis use and the onset of psychosis at a younger age. The association, they say, cannot be explained by chance, and is not related to gender or the use of other drugs. It is, however, related to the amount of cannabis used.

"The clinical importance of this finding is potentially high," Dr. Ana Gonzalez-Pinto from Santiago Apostol Hospital in Vitoria, and colleagues write in the Journal of Clinical Psychiatry, given that cannabis use is extremely prevalent among young people."

The researchers also report that "estimates of the attributable risk suggest that the use of cannabis accounts for about 10 percent of cases of psychosis."

The findings are based on 131 patients ages 15 to 65 years who needed inpatient care for a first psychotic episode during a 2-year period. The subjects were evaluated using the Structured Clinical Interview for DSM-IV Axis I Disorders, and clinical and demographic data were also collected.

The results showed a significant gradual reduction in the age at which psychosis began that correlated with an increased dependence on cannabis. Compared with nonusers, age at onset was reduced by 7, 8.5, and 12 years among users, abusers and dependents, respectively, the researchers report.

In further analysis, the effect of cannabis on age at onset "was not explained by the use of other drugs or by gender," they also note. The finding was similar in the youngest patients, suggesting that this effect was not due to chance.

These results "point to cannabis as a dangerous drug in young people at risk of developing psychosis," Gonzalez-Pinto and colleagues conclude.

SOURCE: Journal of Clinical Psychiatry, August 2008.

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