According to a new study, children and teens who smoke marijuana may be at greater risk of developing depression later on. The study does not make clear, however, whether marijuana itself is to blame.
For the study, researchers used data from more than 50,000 adults in 17 countries taking part in a World Health Organization mental health study.
Overall, a modest association was found between smoking marijuana before age 17 and later depression, with there being a 50% increase in the risk of experiencing an episode of depression after the age of 17.
The link remained even after the researchers controlled for other factors, such as recent marijuana use, smoking and drinking habits and history of mental health issues.
The link was weakened, however, when childhood conduct problems - such as skipping school, fighting and shoplifting - were considered.
According to lead researcher, Dr. Ron de Graf, this could imply that conduct problems may at least partially explain why marijuana use and depression are linked. However, said de Graf, they were lacking data on conduct problems in certain countries, such as New Zealand and South Africa, so they could not make a full analysis of this link.
Another weakness in the study was that the participants were surveyed on a single occasion and had to rely on their memory of when they first smoked marijuana or experienced depression, rather than being followed over time, which would have provided a more accurate assessment of which came first, marijuana use or depression.
While it cannot be determined from this particular study whether marijuana use does in fact cause later depression, it is still possible, said de Graf, that marijuana use has important affects on mental health. More studies are needed, he says, especially those which follow young people over time to see if they develop mental health problems after first using marijuana.
The study appeared online in the June 9, 2010 issue of the American Journal of Epidemiology.