The first study, by doctors in Australia, tracked 1,600 teenage girls for seven years. The research showed that those who used marijuana every day were five times more likely to suffer from depression and anxiety than non-users.

Girls who used the drug at least once every week were twice as likely to develop depression compared to teenagers who didn't use the drug.

A study by Swedish researchers also revealed further evidence that marijuana use can significantly increase the risk of schizophrenia. The study found that 30 percent of the more than 50,000 men who smoked marijuana in the late 1960s developed schizophrenia.

A third study, by British researchers, found that schizophrenia is more likely in people who start using the drug as teenagers. In a study of 1,000 people in their early 20s, one in 10 who used marijuana as a teenager had since been diagnosed with schizophrenia.

As a result of the findings, researchers said that measures are needed to lower frequent and heavy use of marijuana. Furthermore, awareness campaigns are necessary to inform the public about the risk of marijuana use and mental illness.

The study's findings are published in the November 2002 issue of the British Medical Journal.

Arseneault, L., Cannon, M., Poulton, R., Murray, R., Caspi, A., & Moffitt, T. Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. British Medical Journal, 325: 1212-1213.

Patton, G., Coffey, C., Carlin, J., Degenhardt, L., Lynskey, M., & Hall, W. (2002) Cannabis use and mental health in young people: cohort study. British Medical Journal, 325: 1195-1198.

Zammit, S., Allebeck, P., Andreasson, S., Lundberg, I., & Lewis, G. (2002) Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. British Medical Journal, 325: 1199.

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