Seattle–focused study suggests earlier intervention with depressed youths could reduce rate of cannabis–use disorder.
Young people with chronic or severe depression are at elevated risk for developing a problem with cannabis in later adolescence, new research indicates.

The study, led by UW Medicine investigators, interviewed 521 students recruited from four Seattle public middle schools. Researchers used data from annual assessments when students were ages 12 to 15 and then again when they were 18.

The results were published in the journal Addiction.

“The findings suggest that if we can prevent or reduce chronic depression during early adolescence, we may reduce the prevalence of cannabis use disorder,” said lead author Isaac Rhew, research assistant professor of psychiatry and behavioral sciences at the University of Washington School of Medicine.

What researchers called “a 1 standard deviation increase” in cumulative depression during early adolescence was associated with a 50 percent higher likelihood of cannabis–use disorder in the study.

During the past decade, cannabis use among adolescents has surpassed that of tobacco. Cannabis and alcohol are the two most commonly used substances among youth in the United States. They cited one national study showing increases in the prevalence of cannabis–use disorder and alcohol–use disorder in the United States, especially among young adults.

Longitudinal studies of depression and later use of alcohol and cannabis, however, have been mixed. Some show a link, others don’t. Most such studies have assessed adolescent depression at a single point in time – not cumulatively, the researchers noted. Further, previous research has measured substance use differently, ranging from initiation of any use to heavier, problematic use.

The study oversampled for students with depressive and/or conduct problems. The researchers were surprised by data indicating that the prevalence of cannabis– and alcohol–use disorder in this study was notably higher than national estimates, with 21 percent meeting criteria for cannabis–use disorder and 20 percent meeting criteria for alcohol–use disorder at age 18.

What effect the easing of marijuana laws in Washington state had on the youth is unclear. Researchers said it would be informative to conduct a similar study in a state with stricter marijuana laws to understand whether the relationship between depression and later cannabis misuse is similar.

The substance–abuse assessments of 18–year–olds occurred between 2007 and 2010. Washington state legalized medical cannabis in 1998 and its medical cannabis market expanded greatly after 2009, when the U.S. justice department issued a ruling known as the "Ogden Memo." And in 2003, the city of Seattle made cannabis offenses the lowest enforcement priority for police and the city attorney.