Pubdate: Sat, 23 Nov 2002 Source: British Medical Journal, The (UK) Copyright: 2002 The BMJ Contact: http://www.bmj.com/ Details: http://www.mapinc.org/media/60 Authors: George C Patton, Carolyn Coffey, John B Carlin, Louisa Degenhardt, Michael Lynskey, and Wayne Hall Note: LTEs regarding published articles should be submitted via the website's 'rapid response' facility (box at top right of the article in question) or may be emailed to contact address above but must include a subheading to article CANNABIS USE AND MENTAL HEALTH IN YOUNG PEOPLE: COHORT STUDY Papers pp 1199, 1212 George C Patton, professor of adolescent health a, Carolyn Coffey, epidemiologist a, John B Carlin, director of unit b, Louisa Degenhardt, research fellow c, Michael Lynskey, visiting research fellow d, Wayne Hall, professor of bioethics e. a Centre for Adolescent Health, Murdoch Children's Research Institute, Parkville, Victoria 3052, Australia, b Clinical Epidemiology and Biostatistics Unit, Murdoch Children's Research Institute, c National Drug and Alcohol Research Centre, University of New South Wales, Sydney 2052, Australia, d Department of Psychiatry, Washington University School of Medicine, St Louis, MO 63110, USA, e Office of Public Policy and Ethics, Institute for Molecular Bioscience, University of Queensland, Brisbane 4072, Australia Correspondence to: G Patton Objective: To determine whether cannabis use in adolescence predisposes to higher rates of depression and anxiety in young adulthood. Design: Seven wave cohort study over six years. Setting: 44 schools in the Australian state of Victoria. Participants: A statewide secondary school sample of 1601 students aged 14-15 followed for seven years. Main outcome measure: Interview measure of depression and anxiety (revised clinical interview schedule) at wave 7. Results: Some 60% of participants had used cannabis by the age of 20; 7% were daily users at that point. Daily use in young women was associated with an over fivefold increase in the odds of reporting a state of depression and anxiety after adjustment for intercurrent use of other substances (odds ratio 5.6, 95% confidence interval 2.6 to 12). Weekly or more frequent cannabis use in teenagers predicted an approximately twofold increase in risk for later depression and anxiety (1.9, 1.1 to 3.3) after adjustment for potential baseline confounders. In contrast, depression and anxiety in teenagers predicted neither later weekly nor daily cannabis use. Conclusions: Frequent cannabis use in teenage girls predicts later depression and anxiety, with daily users carrying the highest risk. Given recent increasing levels of cannabis use, measures to reduce frequent and heavy recreational use seem warranted. What is already known on this topic Frequent recreational use of cannabis has been linked to high rates of depression and anxiety in cross sectional surveys and studies of long term users Why cannabis users have higher rates of depression and anxiety is uncertain Previous longitudinal studies of cannabis use in youth have not analysed associations with frequent cannabis use What this study adds A strong association between daily use of cannabis and depression and anxiety in young women persists after adjustment for intercurrent use of other substances Frequent cannabis use in teenage girls predicts later higher rates of depression and anxiety Depression and anxiety in teenagers do not predict later cannabis use; self medication is therefore unlikely to be the reason for the association Introduction After increases in cannabis use during the early 1990s, a majority of young people in the United Kingdom, United States, New Zealand, and Australia now use cannabis recreationally. 1 2 Despite the high prevalence of cannabis use, uncertainty persists about its physical and psychological consequences.3 Among the most prominent concerns have been putative links between use of cannabis and mental disorders. A large intake of cannabis seems able to trigger acute psychotic episodes and may worsen outcomes in established psychosis. 4 5 Associations with non-psychotic disorders have received less attention. Yet evidence for an association between cannabis use and depression and anxiety has grown.6 Chronic daily users report high levels of anxiety, depression, fatigue, and their motivation is low.7 In one recent survey of young adults, over a third reported symptoms of anxiety that were associated with cannabis use; young women reported these more commonly.8 Cross sectional associations between cannabis use and depression and anxiety have now been reported in surveys in both adolescents and adults, 9 10 although not all studies have found an association in male participants.11 Questions remain about the level of association between cannabis use and depression and anxiety and about the mechanism underpinning the link. Pre-existing symptoms might raise the likelihood of cannabis use through a mechanism of self medication.12 Alternatively, cannabis use may be more likely in people with a background of social adversity or particular characteristicsfactors that might also raise risks for mental disorders. Cannabis may also carry a direct risk for depression and anxiety. We examined the risks for later depression and anxiety associated with cannabis use in teenagers. Specifically, the study addressed three questions. Firstly, does cannabis use in adolescents predict the development of symptoms of depression and anxiety in young adults? Secondly, do symptoms of depression and anxiety in adolescence predict cannabis use in young adults? Thirdly, is any relation explained by factors such as family background or intercurrent use of other substances? Methods Sample Between August 1992 and December 1998 we conducted a seven wave cohort study of adolescent health in the Australian state of Victoria. The cohort was defined in a two stage cluster sample, in which we selected two classes at random from each of 44 schools drawn from a stratified frame of government run, Catholic, and independent schools (total number of students 60 905). School retention rates to year nine in the year of sampling were 98%. One class from each school entered the cohort in the latter part of the ninth school year (wave 1) and the second class six months later, early in the 10th school year (wave 2). Participants were subsequently reviewed at six month intervals for the next two years (waves 3 to 6), with a final follow up (wave 7) at the age of 20-21, three years after the final school year in Victoria. In waves 1 to 6, participants self administered the questionnaire on Strengths Earlier cohort studies had a limited capacity to address the key questions of this study. One study reported a prospective relation between cannabis use and later depression but started well after the risk period of onset for both.20 Two important studies in adolescence examined either monthly cannabis use or use in the preceding yeardoses that in the light of this study are unlikely to be associated with mental health problems. 21 22 Our close to representative sample, high rates of participation, and frequent measures during participants' teenage years are strengths of this study. A telephone interview strategy was used in data collection in the last wave, and, although prevalence estimates may vary slightly as a result, it is unlikely to have caused a systematic bias in patterns of association. The use of multiple imputation minimised measurement biases arising from missing data during the teenage years, but we did not attempt to adjust for differential participation of young adults. Even though depression and anxiety in teenagers and cannabis use did not predict dropout from the study, the difference in non-responders on other factors (for example, sex or family structure) may have had some bearing on the specification of associations. What the Results Might Mean Possible explanations for the high degree of depression and anxiety found in young women who used cannabis often include underlying characteristics that predispose to both anxiety and depression, self medication of pre-existing depressive symptoms, and an adverse effect of cannabis on mental health.21 The association with cannabis use persisted after adjustment for concurrent use of alcohol, tobacco, and other illicit substances as well as indices of family disadvantagefindings consistent with a more direct relation. We considered self medication with cannabis but found no prospective relation between depression and anxiety in adolescence and later frequent cannabis use, consistent with an earlier report.22 The persistence of associations in the multivariate models and the evidence for a prospective dose-response relation are consistent with a view that frequent use of cannabis in young people increases the risks of later depression and anxiety. Psychosocial mechanismsfor example, the adoption of a countercultural lifestylepossibly underlie the association. Social consequences of frequent use include educational failure, dropout, unemployment, and crimeall factors that may lead to higher rates of mental disorders. Because risks seem confined largely to daily users, however, the question about a direct pharmacological effect remains. Cannabinoid receptors (CB1) are found widely in the central nervous system, with a distribution that is consistent with effects on a wide range of brain functions including memory, emotion, cognition, and movement.23 Cannabis use in young people remains a controversial area, and absence of good data has handicapped the development of rational public health policies.3 These findings contribute to evidence that frequent cannabis use may have a deleterious effect on mental health beyond a risk for psychotic symptoms. Strategies to reduce frequent use of cannabis might reduce the level of mental disorders in young people. Acknowledgments Contributors: GCP was the principal investigator and prepared the manuscript. CC was the study coordinator and contributed to data analysis and manuscript preparation. JBC contributed to the data analysis and manuscript preparation. LD, ML, and WH contributed to the preparation of the manuscript. GCP is the guarantor. 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