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JOURNAL: BMJ. 2002;325:1183-1184, 1195-1198, 1199-1201, 1212-1213
AUTHORS: Louise Arseneault, Stanley Zammit
ABSTRACT: — Three cohort studies published in the British Medical Journal suggest that frequent cannabis use increases the risk of developing depression and schizophrenia in later life.
COMMENTARY: This study suggests that girls who use cannabis as teenagers are more likely to develop anxiety or depressive disorders. Among 1,601 students from 44 secondary schools, frequent cannabis use predicted later depression and anxiety, especially in girls.
By age 20, 60% of participants had used cannabis, and 7% used it daily. After adjustments for use of other substances, daily cannabis use in young women was associated with a more than fivefold increase in risk of later depression and anxiety (odds ratio [OR], 5.6; 95% confidence interval. Weekly or more frequent use as a teenager doubled later risk.
A 1969-1970 survey of 50,087 male Swedish conscripts, aged 18 to 20 years, showed that use of cannabis increased the risk of schizophrenia by 30% (adjusted OR for linear trend. This study also suggests a dose relationship with the development of schizophrenia, because the adjusted OR for using cannabis more than 50 times was 6.7.
"Cannabis use is associated with an increased risk of developing schizophrenia, consistent with a causal relation," write Stanley Zammit, from the University of Wales College of Medicine in Cardiff, the UK, and colleagues. "This association is not explained by use of other psychoactive drugs or personality traits relating to social integration."
Although the authors suggest that the preponderance of evidence is that occasional cannabis use has few harmful effects overall, repeated use is a potentially serious risk to mental health, especially in the presence of other risk factors for schizophrenia. These risks should be recognized in light of current trends to liberalize or even legalize cannabis use.
The third study showed that using cannabis in adolescence increased the likelihood of experiencing symptoms of schizophrenia in adulthood. Those who used cannabis by age 15 years were at greatest risk.
"Our findings suggest that cannabis use among psychologically vulnerable adolescents should be strongly discouraged by parents, teachers, and health practitioners," write Louise Arseneault, from King's College in London, U.K., and colleagues. "Policy makers and law makers should concentrate on delaying onset of cannabis use."