The developing brain is a fragile thing, and that's just one of the reasons why public health experts get so nervous about teenage drug use.

Around the world, marijuana is the most common illicit drug used by adolescents. But even today, we know surprisingly little about what this psychoactive substance does to the adult brain, let alone a developing one.

The first major review on this subject has now shown a link between teenage marijuana use and an increased risk of depression.

Bringing together 11 international studies published from the mid-nineties onwards, the researchers followed the data of 23,000 teenagers right into adulthood.

After accounting for other factors at play, the findings reveal cannabis use before 18 years of age increases the chance of developing depression in adulthood by 37 percent.

In addition, while this habit wasn't linked to anxiety, those teenagers who used cannabis were more than three times more likely to try to kill themselves, although the authors say this calculation is imprecise.

To be clear, the risk of depression and suicidal thoughts here is quite modest. On a population level it's not especially obvious and on a population level, it's only around 7 percent. But that doesn't mean it isn't worth considering, especially given marijuana's popularity among teenagers.

In the US, this roughly translates to more than 400,000 adolescent cases of depression that were linked to cannabis exposure. In Canada, it's more like 25,000 and in the United Kingdom about 60,000.

"Our findings about depression and suicidality are very relevant for clinical practice and public health," says co-author Andrea Cipriani, a psychiatrist at the University of Oxford.

"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."

The findings are supported by some other reviews, which suggests that even in adulthood, marijuana smokers are faced with a moderate risk of developing depression.

The thing is, these are just correlations. So while marijuana use and depression may often keep company, there's no clear evidence that marijuana use directly causes depression.

The explanation is, in all likelihood, far more complex. Cannabis use, for instance, is also associated with other factors that increase risk of depression such as school dropout and unemployment.

Plus, teenagers or adults could be using marijuana to cope with depressive symptoms not necessarily caused by the weed itself.

It's a notoriously difficult subject to research. After all, the moral qualms for testing marijuana on teenagers are manifold.

As a result, most research so far has focused on animals, and while some of these studies have found that marijuana is linked to important changes in the developing brain, we can only conclude so much from these models.

This study has been published in JAMA Psychiatry.