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Pot May Cause Depression, Schizophrenia

Strongest Evidence Yet that Marijuana May Lead to Mental Illness

From the WebMD Archives

Nov. 21, 2002 -- Three newly published studies link frequent marijuana use at a young age to an increased risk of depression and schizophrenia later in life. The studies offer some of the best evidence yet that smoking marijuana can influence the progression of mental illness.

Past research has linked pot smoking with depression and schizophrenia. But it has been unclear whether marijuana use causes psychosis or whether those prone to psychosis self-medicate with the drug. The new studies, published in the Nov. 23 British Medical Journal, suggest a direct link between frequent marijuana use and psychiatric illness that is not explained by self-medication.

"Most [earlier] studies suggested that mental illness in not a result of using marijuana, but the other way around," child and adolescent psychiatrist Joseph M. Rey, MD, PhD, tells WebMD. "These [new] studies do not disprove the self-medication theory completely. But they offer strong support for the explanation that cannabis use causes both schizophrenia and depression."

In an editorial accompanying the studies, Rey notes that it is not yet clear whether marijuana use triggers mental illness in otherwise vulnerable people or if it causes these conditions in people who are not predisposed to them.

In the largest of the newly reported studies, researchers followed about 50,000 Swedish men for 27 years after being drafted for military service. All of the recruits underwent a psychological evaluation, which included questions about drug use, upon entering the service at age 18.

Those who reported smoking marijuana more than 50 times were three times more likely to develop schizophrenia over the next three decades as those who did not use the drug. The association was dose-dependent, with those who smoked pot five to 10 times having only a slightly increased risk. No association was seen between alcohol use and later schizophrenia.

"We cannot be certain that the increased risk we saw is due to cannabis use, but it is the most likely explanation," psychiatrist and lead researcher Stanley Zammit tells WebMD. "It is important to point out that the risk is still quite small. If your lifetime risk of developing schizophrenia is 1% then frequent use of cannabis would increase that risk to 3%."

A second study followed Australian secondary school students for seven years. Researchers found that frequent marijuana was predictive of later depression and anxiety, particularly among teenaged girls.

Roughly 60% of the 1,600 students surveyed had smoked pot by the age of 20 and 7% were daily users. After accounting for use of other substances, daily use was associated with a five-fold increase in later depression and anxiety among young women. But depression and anxiety were not predictive of later marijuana use.

In the final study, researchers assessed data from a study following more than 750 New Zealanders from birth to age 26. Adolescents who were smoking marijuana by the age of 15 had four times the risk for developing schizophrenia in adulthood as those who did not use the drug. The increased risk could not be explained by psychotic tendencies reported in the children at age 11.

"This suggests that there is a direct causal link that cannot be explained by tendency toward mental illness," lead researcher Louise Arseneault, PhD, tells WebMD. "The strength of this study is that these children have been followed since birth. The weakness is that it is a small group, and schizophrenia is a rare disease."

Arseneault says the findings from her study and the others should serve as a wake-up call to those who deny that frequent marijuana use can impact mental health.

"We should try to discourage young people from using cannabis, especially those who might be psychologically vulnerable," she says."

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SOURCES: British Journal of Medicine, Nov. 23, 2002 • Stanley Zammit, psychiatrist, Medical Research Council clinical training fellow, University of Wales College of Medicine at Cardiff • Louise Arseneault, PhD, lecturer, SGDP Research Center, King's College, London • Joseph Rey, MD, PhD, professor of Child and Adolescent Psychiatry, University of Sydney, New South Wales, Australia.
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