The studies were observational, meaning they could not prove that use of the drugs triggered the later depression, researchers said. It it possible that those who were struggling in adolescence were more likely to turn to drugs, they said.
Professor Cipriani said: “Although the size of the negative effects of cannabis can vary between individual adolescents and it is not possible to predict the exact risk for each teenager, the widespread use of cannabis among young generations makes it an important public health issue.
“Regular use during adolescence is associated with lower achievement at school, addiction psychosis and neuropsychological decline,” he said.
Fellow researcher Dr Gabriella Gobbi, from McGill University, said teenagers often thought cannabis was safe because it was derived from plants.
“It’s very important to inform adolescents about the risk and about the kinds of cannabis they use,” she said.
“Today, it is not as in the 80s and 90s, when THC (tetrahydrocannabinol) was about three per cent in joints. Now we have joints of 10, 20, 30 even more per cent and adolescents must be aware of this.”
She also highlighted the dangers of consuming ‘hash cookies’ and other foods containing drugs.
“Many cannabis today, we have edibles. Adolescents buy cookies with THC and cannabidiol with different percentages. They are not aware that edibles have some risks,” she said.
Dr. Joseph Firth, Senior Research Fellow, NICM Health Research Institute at Western Sydney University, said: “Among young adults worldwide, depression is the leading cause of disability, and suicide is the most common cause of death. By linking cannabis as a contributing factor to both of these huge issues for public health, this latest study provides new insights into importance of reducing adolescent cannabis use.
“However, it is important to consider that not all cannabis is equal. In particular, high-THC strains of cannabis are typically associated with more severe impact on mental health – whereas another component of cannabis (known as ‘CBD’) may even attenuate some of the adverse effects.”
He called for further research on the varying affects of different types of cannabis use and the impact of policies which aimed to cut drug use.
“If such schemes actually can produce significant reductions in the incidence of mental illness and suicide among young adults, these efforts could lead to population-scale prevention strategies for tackling the severe mental health issues currently affecting many young adults all over the world,” he said.