Research carried out by scientists investigating depression in early teenage years and the use of marijuana later in life has found a concerning link.
The study is the work of a team from the University of Washington in Seattle, and their findings are published in the journal Addiction.
Depression, also known as clinical depression or major depressive disorder, is a common but serious illness that affects mood, thinking, and behavior.
Depression is more than just feeling a bit sad now and again. It diminishes many aspects of daily living, such as sleeping, eating, working, enjoying hobbies, and socializing.
Major depression is a common mental disorder among teenagers in the United States. National estimates for 2015 suggest that 3 million young people aged between 12 and 17 had experienced "at least one major depressive episode in the past year." This figure represents 12.5 percent of that age group in the U.S.
Teens at risk for marijuana-use disorder
People with cannabis- or marijuana-use disorder often have symptoms of withdrawal when they stop using the substance. In some cases, it can take the form of a severe addiction.
It has been suggested that 30 percent of marijuana users "may have some degree of marijuana-use disorder."
The likelihood of developing marijuana-use disorder is four to seven times higher in people who start using the drug before the age of 18.
Dependence on marijuana develops when the brain adapts to the drug and reduces its own production of, and sensitivity to, similar compounds called endocannabinoid neurotransmitters, which it produces naturally.
The researchers behind the new study note that over the past decade, use of marijuana has overtaken tobacco use among U.S. teenagers. Marijuana and alcohol are now the two most commonly used substances in this age group.
Link between depression, marijuana unclear
The researchers also note that while they often occur together in adolescence, the relationship between depression and marijuana use is not clear.
Studies that have followed young people over a period of time have been done, but the results have been mixed: some have found links between depression and substance use while others have not.
The researchers argue that this could be because those studies have only assessed depression use at a single point in time, and not used a cumulative count. They say that there have also been problems in measuring substance use.
So, the team decided to investigate how cumulative depression in early teens (between the ages of 13 and 15 years) might be related to marijuana-use disorder later on, at age 18.
They recruited 521 students attending four public middle schools in Seattle. The students and their parents underwent "structured in-person interviews" to assess diagnostic symptoms.
Depression linked to marijuana-use disorder
The assessments were carried out at least annually as the students progressed from the 6th to the 9th grade (approximately from age 12 to age 15), and then again at age 18. The authors note that cumulative depression "was defined as the sum of depression symptom counts from grades 7-9."
The results showed that there was a statistically significant link between cumulative depression during the early teenage years and a higher chance of marijuana-use disorder later.
The authors note that a "one standard deviation increase in cumulative depression" during early adolescence was tied to a "50 percent higher likelihood" of marijuana-use disorder at age 18.
"The findings suggest that if we can prevent or reduce chronic depression during early adolescence, we may reduce the prevalence of cannabis-use disorder."
First author Prof. Isaac Rhew, University of Washington School of Medicine
The marijuana-use assessments at age 18 were done between 2007 and 2010, around a decade after Washington state legalized the medical use of marijuana.
The researchers point out that the market for medical marijuana grew considerably following the 2009 Ogden Memo from the federal justice department, which gives "formal guidelines for federal prosecutors in states that have enacted laws authorizing the use of marijuana for medical purposes."
The researchers also comment that, in 2003, the city of Seattle placed marijuana offenses at the bottom of the priority list for law enforcement.
They suggest that it would be interesting to see whether a similar study conducted in a state with stricter marijuana laws would produce the same results.