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Is Marijuana Use Linked To Depression In Kids?

July 7, 2010

Children and teenagers who smoke marijuana may have an increased risk of developing depression, according to a new study.

Several studies have found a link between marijuana use and the risk for depression and other anxiety disorders, but others have also failed to confirm such an association. It is also unclear whether marijuana use itself accounts for the connection seen in some studies.

For the study, researchers used data collected from more than 50,000 adults in 17 countries that took part in a World Health Organization mental-health study.

Researchers found a moderate association between pot use before the age of 17 and the odds of suffering from depression later in life. Early marijuana use was linked to a 50 percent increase in the risk of developing some form of depression after the age of 17.

The link remained when the researchers accounted for other factors, such as self-reported marijuana use, smoking and drinking habits, and history of mental health problems like phobias.

The link did weaken, though, when the investigators factored in childhood conduct problems — like skipping school, fighting, or stealing.

Lead researcher Dr. Ron de Graaf, of the Netherlands Institute of Mental Health and Addiction in Utrecht, said that conduct problems may “partially” explain the relationship between early pot smoking and later depression.

Unfortunately Graaf and his colleagues did not have adequate information on conduct problems from study participants in certain countries, especially in countries where the connection between marijuana use and depression was the strongest, such as South Africa and New Zealand, Graaf told Reuters Health in an email.

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

Other key limitations affected the study as well. Participants were surveyed at a single point rather than over time, which is the best way to gauge which came first — drug use or depression.

Instead, the participants were screened for episodes of depression and then asked to recall when they had first started having them.

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

In the depression group, nine percent said they had smoked marijuana before the age of 17, while seven percent of the comparison group said they had smoked it before the age of 17. In general, men and women who said they used pot before age 17 had a greater chance of having a depression episode at age 17 or older.

This type of study design cannot prove that marijuana itself was the contributing cause to increased depression risk. It may be that young people who are vulnerable to developing depression or other mental health issues are also more likely to use marijuana.

Still, de Graff said, it is possible that children’s and teenagers’ still-developing brains may be more vulnerable to any direct toxic effects of marijuana use.

“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 — particularly ones that follow young people over time to see whether marijuana use does precede the development of depression and other mental health problems in some children, said de Graaf.

The study is published in the American Journal of Epidemiology.

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