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The findings applied to heavy cannabis users
Frequent cannabis use can trigger depression, a study suggests.
Researchers have also found further evidence the drug can significantly increase the risk of schizophrenia.
The risks are outlined in three papers in the latest issue of the British Medical Journal. Researchers say their findings highlight the need for measures to reduce frequent and heavy use of cannabis.
The first paper, by doctors in Australia, found frequent cannabis use among teenage girls in particular can trigger depression.
Their seven-year study of 1,600 teenage girls found girls who used the drug everyday were five times more likely to become depressed and suffer from anxiety compared to those who did not use the drug.
Those who used the drug at least once every week were twice as likely to develop depression compared to non-users.
A second study, by doctors in Sweden, confirmed previous research suggesting that cannabis can increase the risk of developing depression.
Their study of more than 50,000 men found those who had smoked the drug in the late 1960s were 30% more likely to have developed schizophrenia.
The authors said their results suggested that as many as one in eight cases of schizophrenia in the UK could be prevented by stopping people from using cannabis.
The third study, by British scientists, suggests the risks of developing schizophrenia are highest for those people who use the drug when they are a teenager.
Their study of more than 1,000 people in their early twenties in New Zealand suggested that one in 10 people who used cannabis as a teenager have since been diagnosed with schizophrenia.
All of the researchers said the ill-effects were linked to cannabis and not to any other drugs.
They also said there was little evidence to suggest that occasional use of cannabis had a similar effect.
Further research
In an accompanying editorial Joseph Rey, professor of child and adolescent psychiatry at the University of Sydney, said the studies backed up previous research.
"These findings strengthen the argument that use of cannabis increases the risk of schizophrenia and depression."
But he added that further research is needed.
"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."
Education campaign
The UK charity Rethink, formerly the National Schizophrenia Fellowship, said the findings highlighted the need for a public education campaign on the risks of using cannabis.
Cliff Prior, its chief executive, said: "The research highlights how cannabis can be one of the triggers for severe mental illnesses like schizophrenia and depression.
"It shows why it is so important that more work of this kind is done so that people with severe mental illness can have the best chance of recovering a meaningful quality of life."
He added: "Cannabis is not a risk-free drug. The public needs to understand the potential dangers of triggering mental illness."
Marjorie Wallace, chief executive of mental health charity SANE, added: "The most worrying revelation of these studies is not just the immediate triggering of hallucinations but that cannabis can lead to psychotic symptoms and depression in later life."
http://news.bbc.co.uk/2/hi/health/2498493.stm
Frequent cannabis use can trigger depression, a study suggests.
Researchers have also found further evidence the drug can significantly increase the risk of schizophrenia.
The risks are outlined in three papers in the latest issue of the British Medical Journal. Researchers say their findings highlight the need for measures to reduce frequent and heavy use of cannabis.
The first paper, by doctors in Australia, found frequent cannabis use among teenage girls in particular can trigger depression.
Their seven-year study of 1,600 teenage girls found girls who used the drug everyday were five times more likely to become depressed and suffer from anxiety compared to those who did not use the drug.
Those who used the drug at least once every week were twice as likely to develop depression compared to non-users.
A second study, by doctors in Sweden, confirmed previous research suggesting that cannabis can increase the risk of developing depression.
Their study of more than 50,000 men found those who had smoked the drug in the late 1960s were 30% more likely to have developed schizophrenia.
The authors said their results suggested that as many as one in eight cases of schizophrenia in the UK could be prevented by stopping people from using cannabis.
The third study, by British scientists, suggests the risks of developing schizophrenia are highest for those people who use the drug when they are a teenager.
Their study of more than 1,000 people in their early twenties in New Zealand suggested that one in 10 people who used cannabis as a teenager have since been diagnosed with schizophrenia.
All of the researchers said the ill-effects were linked to cannabis and not to any other drugs.
They also said there was little evidence to suggest that occasional use of cannabis had a similar effect.
Further research
In an accompanying editorial Joseph Rey, professor of child and adolescent psychiatry at the University of Sydney, said the studies backed up previous research.
"These findings strengthen the argument that use of cannabis increases the risk of schizophrenia and depression."
But he added that further research is needed.
"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."
Education campaign
The UK charity Rethink, formerly the National Schizophrenia Fellowship, said the findings highlighted the need for a public education campaign on the risks of using cannabis.
Cliff Prior, its chief executive, said: "The research highlights how cannabis can be one of the triggers for severe mental illnesses like schizophrenia and depression.
"It shows why it is so important that more work of this kind is done so that people with severe mental illness can have the best chance of recovering a meaningful quality of life."
He added: "Cannabis is not a risk-free drug. The public needs to understand the potential dangers of triggering mental illness."
Marjorie Wallace, chief executive of mental health charity SANE, added: "The most worrying revelation of these studies is not just the immediate triggering of hallucinations but that cannabis can lead to psychotic symptoms and depression in later life."
http://news.bbc.co.uk/2/hi/health/2498493.stm
