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 gpatton@cryptic.rch.unimelb.edu.au

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.

Footnotes

Editorial by Rey and Tennant

Funding: National Health and Medical Research Council and Victorian Health
Promotion Foundation.

Competing interests: None declared.

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