CANNABIS AND MENTAL HEALTH

More Evidence Establishes Clear Link Between Use Of Cannabis And
Psychiatric Illness

Papers pp 1195, 1199, 1212

In the 1990s the use of cannabis increased much among young people so that
it is now becoming more common than tobacco smoking in some countries.1-2
The ready availability of the drug, the increasing social disapproval of
cigarette smoking, stern drink driving laws, and perceptions that cannabis
is safe or less harmful than cigarettes or alcohol may explain these
changes. The increase in use is of concern because cannabis may be a
gateway to other drugs,3 and it may cause psychiatric illnesses.

The link between cannabis and psychosis is well established, and recent
studies have found a link between use of marijuana and depression.4-7 Does
cannabis cause these conditions, or do patients use cannabis to relieve
their distress?

The explanation most accepted is that cannabis triggers the onset or
relapse of schizophrenia in predisposed people and also exacerbates the
symptoms generally. 4 5 Establishing direction of causality is difficult
and is most appropriately assessed in non-clinical samples, but a low
incidence of the illness and the fact that most drug users take other drugs
in addition to cannabis create methodological problems and explain the
dearth of reliable evidence.

The study often quoted in support of the causal hypothesis examined the
incidence of schizophrenia in more than 50 000 Swedish conscripts followed
up for 15 years.8 It showed that use of marijuana during adolescence
increased the risk of schizophrenia in a dose-response relation.

Questions have, however, remained about the validity of the diagnosis, the
possible causal role of other drugs, and prodromal symptoms of
schizophrenia that might have led to the use of cannabis, rather than
cannabis triggering the psychosis. 4 5

A longer follow up and reanalysis of this cohort published in this issue (p
1199) confirms the earlier findings and clarifies that cannabis, and not
other drugs, is associated with later schizophrenia and that this is not
explained by prodromal symptoms.9 In a similar vein, a three year follow up
of a Dutch cohort of 4045 people free of psychosis and 59 with a baseline
diagnosis of psychotic disorder showed a strong association between use of
cannabis and psychosis.10 Length of exposure to use of cannabis predicted
the severity of the psychosis, which likewise was not explained by use of
other drugs.

Participants who showed psychotic symptoms at baseline and used cannabis
had a worse outcome, which also implies an additive effect. In a New
Zealand cohort, individuals who had used cannabis three times or more by
age 15 or 18 were not more likely to have schizophreniform disorder at age
26 (p 1212), although they showed an increase in "schizophrenia symptoms"
(but not schizophrenia).11 The meaning of "schizophrenia symptoms" requires
clarification to interpret these results.

The evidence in relation to depression is growing.

A 15 year follow up of an adult community sample of 1920 participants in
the United States showed that use of cannabis increased the risk of major
depression at follow up fourfold.7 Use of cannabis was specifically
associated with an increase in suicidal ideation and anhedonia.

Similar findings in an Australian study reported in this issue (p 1195)
show a dose-effect relation between the use of cannabis and anxiety or
depression in a large cohort of 14-15 year olds followed for seven years.12
This is reflected in higher rates of anxiety or depression according to the
frequency with which cannabis was used. The link is stronger for young
women than young men in this cohort, although sex differences have not been
found in other studies. 6 7 Baseline depression did not predict later
marijuana use in either study and therefore does not support the self
medication hypothesis. The study in the New Zealand cohort did not find an
association between cannabis use at age 15 and depressive disorder at age
26. The authors found, however, that young people who had used cannabis
three times or more by age 18 were more likely to have a depressive
disorder at age 26, even after use of other drugs was controlled for.

Although the number of studies is small, these findings strengthen the
argument that use of cannabis increases the risk of schizophrenia and
depression, and they provide little support for the belief that the
association between marijuana use and mental health problems is largely due
to self medication. Whether the use of cannabis triggers the onset of
schizophrenia or depression in otherwise vulnerable people or whether it
actually causes these conditions in non-predisposed people is not yet
resolved. Further, it cannot be assumed that mechanisms are the same for
both conditions (cannabinoids have effects on a variety of neurotransmitter
systems) or at different developmental stages.

For example, although evidence shows that mental disorder leads to the use
of cannabis among adolescents, the reverse seems true in early adulthood.13

The shown dose-response relation for both schizophrenia and depression
highlights the importance of reducing the use of cannabis in people who use
it. It was estimated that lack of exposure to cannabis would have reduced
the incidence of psychosis requiring treatment by as much as 50% in the
Dutch cohort,10 and is similarly reflected in the Swedish cohort, showing
that the use of cannabis increased the risk of schizophrenia by 30%.9 This
large effect is surprising and not yet reflected in an increased incidence
of schizophrenia in the population. If true, the use of cannabis will
contribute to more episodes or new cases of the illnessfood for thought for
both clinicians and legislators.

Joseph M Rey, professor of child and adolescent psychiatry.

University of Sydney, Coral Tree Family Service, PO Box 142, North Ryde,
NSW 1670, Australia (jrey@doh.health.nsw.gov.au)

Christopher C Tennant, professor of psychiatry.

University of Sydney, Royal North Shore Hospital, St Leonard's, NSW 2065,
Australia (ctennant@doh.health.nsw.gov.au)

Footnotes

Competing interests: None declared.

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