Brain development is a fragile thing, and this is just one of the reasons why public health experts are concerned about teen drug use.
Around the world, marijuana is the most common illicit drug used by teenagers. But even today, we know surprisingly little about the effects of this psychoactive substance on the adult brain, let alone on the developing one.
The first major review on this topic has now shown a link between marijuana use among adolescents and an increased risk of depression.
Bringing together 11 international studies published from the mid-1990s, researchers tracked data from 23,000 adolescents to adulthood.
After taking into account other factors at play, the results show that cannabis use before the age of 18 increases by 37% the chances of developing depression in adulthood.
In addition, although this habit is not related to anxiety, teenagers who used cannabis were more than three times more likely to commit suicide, although the authors say this calculation is imprecise.
To be clear, the risk of depression and suicidal thoughts is quite modest here. At the population level, this is not particularly evident and at the population level, it is only about 7%. But that does not mean it's not worth considering, especially given the popularity of marijuana among teens.
In the United States, this translates into more than 400,000 cases of adolescent depression related to exposure to cannabis. In Canada, it is more or less 25,000 and in the UK about 60,000.
"Our findings on depression and suicidality are very relevant to clinical practice and public health," says co-author Andrea Cipriani, a psychiatrist at Oxford University.
"Although the magnitude of the negative effects of cannabis may vary from one adolescent to another and that it is impossible to predict the exact risk for each adolescent, the generalized use cannabis among younger generations makes it an important public health problem. "
The findings are corroborated by other reviews, which suggest that even in adulthood, marijuana smokers face a moderate risk of developing depression.
The fact is that these are just correlations. Thus, although marijuana use and depression often occur in the company, there is no clear evidence that marijuana use directly causes depression.
The explanation is likely much more complex. Cannabis use, for example, is also associated with other factors that increase the risk of depression, such as dropping out of school and unemployment.
In addition, teens or adults may use marijuana to deal with depressive symptoms that are not necessarily caused by the weed itself.
This is a subject notoriously difficult to search. After all, moral scruples for testing marijuana on teenagers are multiple.
As a result, most research to date has focused on animals. Although some of these studies have shown that marijuana is linked to significant changes in brain development, we can only conclude from these models.
This study was published in JAMA Psychiatry.