Regular Cannabis Use Linked To Mental Illness
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CANNABIS LINK TO MENTAL ILLNESS STRENGTHENED
By Emma Young
November 21, 2002
The link between regular cannabis use and later depression and schizophrenia
has been significantly strengthened by three new studies.
The studies provide "little support" for an alternative explanation -- that
people with mental illnesses self-medicate with marijuana -- according to
Joseph Rey and Christopher Tennant of the University of Sydney, who have
written an editorial on the papers in the British Medical Journal.
One of the key conclusions of the research is that people who start smoking
cannabis as adolescents are at the greatest risk of later developing mental
health problems. Another team calculates that eliminating cannabis use in
the UK population could reduce cases of schizophrenia by 13 per cent.
Until now, say Rey and Tennant, there was "a dearth of reliable evidence" to
support the idea that cannabis use could cause schizophrenia or depression.
That lack of good evidence "has handicapped the development of rational
public health policies," according to one of the research groups, led by
George Patton at the Murdoch Children's Research Institute in Melbourne,
The works also highlights potential risks associated with using cannabis as
a medicine to ease the symptoms of muscular sclerosis, for example.
Patton's team followed over 1600 Australian school pupils aged 14 to 15 for
seven years. Daily cannabis use was associated with a five-fold increased
risk of depression at the age of 20. Weekly use was linked to a two-fold
increase. The regular users were no more likely to have suffered from
depression or anxiety at the start of the study.
The reason for the link is unclear. Social consequences of frequent cannabis
use include educational failure and unemployment, which could increase the
risk of depression. "However, because the risk seems confined largely to
daily users, the question about a direct pharmacological effect remains,"
In separate research, a team led by Stanley Zammit at the University of
Cardiff, UK, evaluated data on over 50,000 men who had been Swedish military
conscripts in 1969 and1970. This group represents 97 per cent of men aged 18
to 20 in the population at that time.
The new analysis revealed a dose-dependant relationship between the
frequency of cannabis use and schizophrenia. This held true in men with no
psychotic symptoms before they started using cannabis, suggesting they were
Finally, researchers led by Terrie Moffitt at King's College London, UK,
analysed comprehensive data on over 1000 people born in Dunedin, New Zealand
in 1972 and 1973.
They found that people who used cannabis by age 15 were four times as likely
to have a diagnosis of schizophreniform disorder (a milder version of
schizophrenia) at age 26 than non-users.
But when the number of psychotic symptoms at age 11 was controlled for, this
increased risk dropped to become non-significant. This suggests that people
already at greater risk of later developing mental health problems are also
more likely to smoke cannabis.
The total number of high quality studies on cannabis use and mental health
disorders remains small, stress Rey and Tennant. And it is still not clear
whether cannabis can cause these conditions in people not predisposed by
genetic factors, for example, to develop them.
"The overall weight of evidence is that occasional use of cannabis has few
harmful effects overall," Zammit's team writes. "Nevertheless, our results
indicate a potentially serious risk to the mental health of people who use
cannabis. Such risks need to be considered in the current move to liberalise
and possibly legalise the use of cannabis in the UK and other countries."
Journal references: British Medical Journal (vol 325, p1195, p1199, p1212,
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