NEW YORK – Teens and children who smoke marijuana may have a heightened risk of depression, a new study shows. Whether the drug itself is to blame is still unclear.

Several studies have shown an association between marijuana and increase risk of depression and anxiety disorders, but most have failed to confirm such a link. It remains unclear as to whether marijuana use or some other factor raises the risk in some studies.

In the new study that was published in the American Journal of Epidemiology, researchers used data from more than 50,000 adults in 17 countries taking part in a World Health Organization (WHO) mental-health study.

The researchers found a modest association between marijuana smoking before the age of 17 and the odds of suffering from depression later in life. Through all the countries, early marijuana use was liked to a 50 percent increase in the risk of developing depression after the age of 17.

The connection stayed when researchers accounted for other factors, including participants' self-reported recent marijuan use, smoking and drinking habits, and history of mental problems such as phobias and anxiety.

The marijuana-depression link was weakened when investigators factored in childhood conduct problems such as skipping school, getting into fights and shoplifting.

The implication is that conduct problems could explain the causality between early pot smoking and later depression, lead researcher Dr. Ron dr Graaf, of the Netherlands Institute of Mental Health and Addiction in Utrecht said in an email.

De Graaf said, unfortunately, that he and his team lacked information on conduct problems from study participants in certain areas, including some of those where the connection between early pot use and depression were strongest like New Zealand and South Africa.

The full extent of that childhood problems may explain the link between marijuana and depression is still not clear.

Other key limitations were present in the study as well. Some participants were surveyed at a single point instead of over time, which is the best way to gauge which came first, drugs or depression.

The study participants were, instead, screened for spells of depression which are persistent feelings of sadness and other symptoms such as appetite change and sleep problems, that lasted for at least two weeks. They were then asked to recall when they first started experiencing episodes.

Those who were 17 years or older when the depression hit, they were considered “cases”. Across the countries studied, 9,647 participants fit the definition and were compared to the 41,000 plud men and women who had no current or past depression.

Of the group of depressed, 9 percent claimed that they had smoked marijuana before that age of 17, while the same was true of 7 percent o the comparison group. Men and women who said they had smoked pot before the age of 17 had a greater chance of a depression episode at 17 or later.

This study type cannot prove outright that marijuana contributed to the increased risk of depression. It may be, for instance, that young folks who are vulnerable to developing depression or other mental health problems are also more likely to use marijuana.

Still, according to de Graaf, it is possible that with other substances, like alcohol, kids' and teens' still-developing brains may be more vulnerable to any direct toxic effects of marijuana use.

Past studies have linked heavy marijuana use, especially with teens, to an increase in schizophrenia-like psychosis.

"Early cannabis (marijuana) use may have important consequences for later mental health," de Graaf said. "We know now -- also from other studies -- that cannabis use is not without negative consequences."

More studies are still needed, according to de Graaf. Ones that follow young people over time to see whether marijuana use does pecede the development of mental health problems in some kids will be vital.