Cannabis link to mental illness strengthened
23:01 21 November 02
NewScientist.com news service
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
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, Australia.
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," says Patton.
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 not self-medicating.