Teenage cannabis users more likely to suffer from depression later in life
Teenagers who use cannabis have a higher risk of developing depression and suicidal thoughts as young adults and should be made aware of those risks by parents and doctors, scientists said.
About 7pc of cases of adolescent depression could be averted if cannabis use was eliminated, according to an analysis of data on mental illness among young people in the US, Britain and Canada who used the drug in their teens.
"Although the size of the negative effects of cannabis can vary ... and it is not possible to predict the exact risk for each teenager, the widespread use of cannabis among the young generations makes it an important public health issue," said Andrea Cipriani, a professor of psychiatry at Oxford University in England, who co-led the work.
"For teenagers, there may be an effect which is biological, with some consequences which can be devastating. It is a vulnerable period of development of the brain - [so] exposing young teenagers to cannabis is likely to increase the risk of depression."
Cannabis is the most commonly used recreational drug by teenagers worldwide.
In Canada, more than 20pc of teenagers aged 15 to 19 said they had used it in the past year.
In England, for those aged 11 to 15, 4pc said they had used cannabis in the past month.
The results suggested that, if cannabis use were eliminated, there would be an estimated 400,000 fewer cases of depression in 18 to 34-year-olds in the US, 25,000 fewer in Canada and about 60,000 fewer in Britain, the researchers said.
"It's a big public health and mental health problem," Ms Cipriani said.
"Adolescents should be made aware of the risk."
The study, published in the journal 'JAMA Psychiatry' and co-led by Ms Cipriani and researchers at McGill University in Canada, was a systematic analysis pooling the best available evidence.
It included 23,317 people from 11 international studies, and looked at depression, anxiety and having suicidal thoughts in young adults.
Independent specialists asked to comment on the study said its findings were robust and important.
"Among young adults worldwide, depression is the leading cause of disability, and suicide is the most common cause of death," said Joseph Firth, a specialist at Australia's Western Sydney University.
By showing cannabis as a contributing factor to both, he said, the study showed the importance of seeking ways to reduce teenage cannabis use.