Parents should not be complacent about the risks of teenagers using cannabis, experts are warning.
UK and Canada researchers said they had found “robust” evidence linking one in 14 cases of depression in the under-35s to use of the drug in adolescence, although there was no definitive proof.
They said the findings should act as a warning to families who saw cannabis use as part of the growing-up process.
The team added that the developing brain was particularly susceptible.
The researchers – from University of Oxford and Montreal’s McGill University – said cannabis use in the young was an “important public health issue”.
Around one in nine young adults and teenagers use the drug each year in England and Wales.
Report author Prof Andrea Cipriani conceded some parents had a relaxed attitude to the drug, but added the evidence was clear.
“This is important information for parents and teenagers. The risk is modest, but it can have a devastating impact.”
How risky is cannabis?
This is the first time the actual risk has been quantified in this way.
The team looked at 11 previous studies, covering more than 23,000 young people.
Young people who had already shown signs of depression or had a family history of the condition were excluded.
It found cannabis use before 18 increased the chance of an individual developing depression in young adulthood – defined as before the age of 35 – by 37%.
How many cases could be prevented?
The researchers believe that about 7% of cases – one in 14 – could be avoided in young adulthood if people did not use cannabis in their teenage years.
Given that around 8% of this age group has depression, that would equate to 60,000 cases in the UK, 25,000 in Canada and more than 400,000 in the US being prevented.
The study also looked at whether there was an association with anxiety and suicide attempts.
There was an increased risk of anxiety developing in young adulthood, but it was not considered statistically significant.
Meanwhile, those who used cannabis in their teenage years were three times more likely to try to kill themselves – although the data on that trend was not considered robust enough to draw a firm conclusion.
So is this proof?
No. The researchers were unable to prove cannabis use was definitively causing depression.
Instead, they could only say it looked to be a strong link.
This is because of the figures they found, but also what is known about the impact cannabis has on the developing brain.
Evidence has suggested that the drug affects the parts of brain that govern rational and emotional thinking as well as serotonin levels that influence mood.
To prove the link, trials would have to be conducted on young people – something that will never be done because it is unethical.
King’s College London psychiatrist Prof Sir Robin Murray said the research could not be considered conclusive, but acknowledged the findings were “probably correct”.
He said the risk of developing depression was smaller than what has already been established for schizophrenia-like psychosis.
He also said it was likely that any risk would be also down to the amounts of cannabis consumed and the strength – something this study was unable to unpick.