Children and teenagers who smoke marijuana may have a somewhat heightened risk of developing depression, a new study suggests - though whether the drug itself is to blame is not clear.

Several studies have found an association between marijuana use and increased risks of depression and anxiety disorders, but some others have failed to confirm such a link. Moreover, it has been unclear whether marijuana use itself, or some other factor, accounts for the connection seen in some studies.

For the new study, published in the American Journal of Epidemiology, researchers used data collected from more than 50,000 adults in 17 countries taking part in a World Health Organization mental-health study.

Overall, the researchers found a modest association between marijuana smoking before the age of 17 and the odds of suffering depression later on. Across the countries, early marijuana use was linked to a 50 percent increase in the risk of developing a depression “spell” after age 17.

The connection remained when the researchers accounted for a number of other factors, including participants’ self-reported recent marijuana use, smoking and drinking habits, and history of mental health problems like phobias and anxiety.

The strength of the marijuana-depression link did weaken, however, when the investigators factored in childhood conduct problems - like skipping school, getting into fights and shoplifting.

The implication is that such conduct problems may “partially” explain the relationship between early pot smoking and later depression, lead researcher Dr. Ron de Graaf, of the Netherlands Institute of Mental Health and Addiction in Utrecht, told Reuters Health in an email.

Unfortunately, de Graaf said, he and his colleagues lacked information on conduct problems from study participants in certain countries - including some of those nations where the connection between early pot use and depression was strongest, such as New Zealand and South Africa.

So the full extent to which childhood behavioral problems may explain the pot-depression link is not clear.

The study has other key limitations as well. One is that participants were surveyed at a single point rather than followed over time, which is the best way to gauge which came first - the drug use or the depression.

Instead, the study participants were screened for depression “spells” - persistent feelings of sadness and other symptoms, such as appetite changes and sleep problems, that lasted for at least two weeks - then asked to recall when they had first started having such episodes.

Those who said they were 17 or older when their depression first hit were considered “cases.” Across the countries, 9,647 participants met that definition and were compared with the 41,000-plus men and women who had no current or past depression.

Of the depression group, 9 percent said they had smoked marijuana before the age of 17, while the same was true of 7 percent of the comparison group. In general, men and women who said they had used pot before age 17 had a greater chance of having a depression episode at age 17 or later.

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

Still, de Graaf said, it is possible that as with other substances, such as alcohol, children’s and teenagers’ still-developing brains may be more vulnerable to any direct toxic effects of marijuana use.

Some past studies have linked heavy marijuana use, particularly early in life, to an increased risk of 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, however, according to de Graaf - particularly ones that follow young people over time to see whether marijuana use does precede the development of mental health problems in some kids.

SOURCE: American Journal of Epidemiology, online June 9, 2010.