A rigorous new meta-study has for the first time examined the correlation between depression or suicide in young adulthood, and marijuana usage in adolescence. The research concludes there is a statistically significant association between teen marijuana use and depression in later life, but experts suggest caution in determining if the relationship is causal.

We know that marijuana consumption during adolescence is certainly not harmless. A great deal of research has confidently suggested the drug does disrupt the healthy growth of a young developing brain. What this all actually means in terms of any permanent damaging psychological effects is still unclear.

Much research has looked at the connection between marijuana use and significant mental health issues such as schizophrenia, however, the long-term mood modulating effects of the drug are still relatively unknown. This new meta-study collected data from 11 different international studies, collecting information on over 23,000 subjects.

"While the link between cannabis and mood regulation has been largely studied in preclinical studies, there was still a gap in clinical studies regarding the systematic evaluation of the link between adolescent cannabis consumption and the risk of depression and suicidal behavior in young adulthood," explains Gabriella Gobbi, one of the researchers from McGill University working on the project. "This study aimed to fill this gap, helping mental health professionals and parents to better address this problem."

The results revealed that individuals who used marijuana before the age of 18 were 37 percent more likely to develop depression by the age of 35. The numbers relating to suicide risk were even starker, with the researchers suggesting smoking marijuana before the age of 18 was associated with 3.5 times higher risk of attempting suicide in young adulthood.

"When we started this study we expected depression to be a factor attributable to cannabis consumption, but we were quite surprised about suicide behavior rates. Indeed, a significant percentage of suicidal attempts are attributable to cannabis," says Gobbi.

Many experts, while suggesting the research is competent and strong, urge caution when trying to attribute causality. The research is resolutely observational and chock full of limitations. There is no clarity on the amount of marijuana a young person is smoking, the strength of that marijuana, what age they commenced consumption, or whether other drugs such as alcohol and tobacco were also being consumed.

"It's also important to note that these results don't tell us if the effects are specifically due to cannabis use during teenage years," says Lindsey Hines, a scientist from the University of Bristol who did not work on this new study. "It may be that people who were smoking cannabis as a teenager have carried on smoking cannabis as adults, which may explain some of the relationship to mental health."

Another question hovering over the research is that of the relative risk between adolescent marijuana use and equivalent adolescent alcohol or tobacco use. A striking recent study from the University of Illinois revealed heavy alcohol consumption in adolescence caused fundamental changes in the wiring of a young brain that made individuals much more susceptible to psychological problems later in life. Another study revealed heavy drinking during adolescence can result in a smaller hippocampus growth, permanently affecting learning and decision-making.

All of this ultimately suggests that, while marijuana is still primarily illegal in most parts of the world, it is unclear whether it's more damaging to a growing young brain than more commonly consumed, and legal, alcoholic drinks. More research is certainly needed to help better clarify the risk and actual damage that marijuana can wreak on young developing brains, so while we can be sure that marijuana is not harmless to an adolescent brain, we do not yet know exactly how harmful it actually is.

The new study was published in the journal JAMA Psychiatry.