Depression directly affects one in six adults over the course of a lifetime – and everything from pollution and artificial light to the bacteria that live in our gut could be (at least partly) to blame.

Now, a new paper published in JAMA Psychiatry also links the mental illness to cannabis smoking as a teenager.

Previous research has suggested that cannabis use moderately increases an individual's risk of developing depression. While others have found no significant association when other variables are taken into consideration. Still others suggest cannabis use may actually decrease symptoms.

The result is a mixed bag of results with no clear consensus of how cannabis affects mental health, at least as far as depression is concerned.

Researchers from the University of Oxford, UK, and McGill University, Canada, have analyzed 11 international studies examining the effects of marijuana use in under-18s and published since the mid-90s. These studies were chosen from 3,142 articles investigating correlations between drug use in adolescence and mental health in later life. Combined, they involved more than 23,000 people.

"We looked at the effects of cannabis because its use among young people is so common, but the long-term effects are still poorly understood," Andrea Cipriani, NIHR Research Professor of Psychiatry at the University of Oxford, said in a statement.

"We carefully selected the best studies carried out since 1993 and included only the methodologically sound ones to rule out important confounding factors, such us premorbid depression."

Their conclusion – one in every 14 cases of depression in adults under 35 could be averted if teens avoided cannabis. In practical terms, that is 400,000 diagnoses of depression in the US, 25,000 diagnoses in Canada, and 60,000 diagnoses in the UK. What's more, smoking cannabis before 18 was associated with a 350 percent increased risk of attempting suicide.

The study authors suggest this correlation may be linked to the psychoactive ingredient in marijuana, Tetrahydrocannabinol, or THC. Animal studies have found an association between adolescent exposure to THC and the development of depressive disorders in adulthood, possibly because it actually alters the physiological neurodevelopment of teenage brains.

It's worth stressing that these are associations, revealing an interesting correlation but not (necessarily) causation. For example, it might not be that cannabis use causes depression but a propensity to develop depression increases the likelihood a person will take up cannabis in the first place. Alternatively, there may be a third factor or factors (perhaps genetic or environmental) that correlate positively to both cannabis use and depression.

Also worth noting is the fact that the studies did not take into consideration participants' use of other drugs or the amount and strength of the cannabis they were using, all of which could be affecting the results and the scale of risk involved in smoking cannabis.

Finally, while the results suggest on a society-wide scale, the problem is widespread, the risk for the individual is relatively modest.

"Our findings about depression and suicidality are very relevant for clinical practice and public health," Cipriani continued.

"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 the young generations makes it an important public health issue."

To sum up, there may be an association between cannabis use in adolescence and depression but more research is needed to confirm and clarify why exactly this is.