Does cannabis cause depression in teenagers?

The global attitude towards cannabis has been changing, drifting more towards exploring its potential medicinal uses and legalizing it.

Eight countries, including Canada, Uruguay, Mexico and Thailand, and 22 states in the US have legalized recreational marijuana, and around 50 countries have legalized it for medicinal use. Many other countries are currently pushing their laws in that direction.

But just like with tobacco and alcohol, legalization doesn’t mean the drug is not harmful.

Marijuana is also one of the most used substances among teenagers around the world. In the US, more 2.5 million teens casually use cannabis, according to researchers from Columbia University in New York, and cannabis usage among youth has increased over the last decades.

That’s why the trend towards legalization and medicinal use has raised alarms, particularly about the potential health risks in adolescents.

Drugs and a developing brain

Although it can be tricky to tell when adolescence stops, it’s clear that it is a period that comes with many biological changes, including changes in the brain.  

Those changes make it even harder to understand how cannabis can affect teenagers’ minds. 

During adolescence, the brain is in development up to about the mid-20s, according to the US’ National Institute of Mental Health.

During this time, there’s major development and fine-tuning in areas of the brain related to handling emotions, coping with stress, rewards and motivation, decision-making, thinking before acting, controlling impulses and reasoning, just to name a few. There’s also an increase in white matter and a decrease in gray matter during adolescence, which makes different brain regions communicate faster and more efficiently. 

It’s a hard knock, life for teenagers. Not only do their bodies go through drastic changes, but they often struggle with issues like identity, social pressure, getting good grades, family dynamics and many other things.  

All these changes and pressures can make teens more likely to have mental health issues, like anxiety and depression, and they can lead to their using substances like marijuana to cope, according to the US Substance Abuse and Mental Health Services Administration. The problem is that using marijuana can also make those mental health conditions worse in the long run. 

Because the brain is developing in this stage, it is also particularly vulnerable to substances like alcohol, tobacco, marijuana and other drugs acting on it. These substances have been shown to change or delay some of the developments usually taking place during adolescence, according to the American Academy of Child and Adolescent Psychiatry. 

In the case of cannabis, there’s increasing evidence that it modifies the brain of teenagers. 

The evidence regarding cannabis and depression 

Marijuana usage has been linked to difficulties with thinking and solving problems, memory and learning, and with reduced coordination and concentration, according to the US Centers for Disease Control and Prevention (CDC). It’s still not clear if these problems persist after stopping cannabis use. 

Research has also shown associations between cannabis use and mental health problems like anxiety and depression. It’s also more likely for people that use cannabis to have psychotic episodes. 

A study, published in JAMA, the Journal of the American Medical Association, earlier this month, looked into teens who occasionally used cannabis in the last 12 months. The study analyzed the responses of almost 70,000 teenagers to the 2019 National Survey on Drug Use and Health.  

The study found that, compared to non-users, those that did use marijuana but didn't meet the criteria for addiction reported two to four times more mental health problems like depression, suicidal thoughts, slower thinking and difficulty concentrating.  

This could suggest there's an association between marijuana use and mental health issues, but it's still unclear whether one directly causes the other. 

Another recent study published in JAMA Psychiatry found that marijuana use in teens was also associated with an increased risk of developing depression and suicidal thoughts later in life. 

However, a 2022 study published in the Journal of Psychopharmacology showed that teenagers that used cannabis weren’t more likely to develop mental health problems like depression or anxiety compared to adults that used cannabis. Only teenagers with cannabis addiction had worse mental health.

Is cannabis the cause?

Correlation does not equal causation and as in the case of the chicken and the egg, it's hard to tell whether the use of cannabis in adolescents is the cause of the higher association with depression and other mental health issues, or whether teens with these issues are more likely to use cannabis.

A 2020 study published in Frontiers in Psychiatry reviewed the evidence on cannabis and the teenage brain and concluded that because of the way many of the available studies, so-called cross-sectional studies, were designed, we don’t know much about the nature of the relationship between cannabis use and mental health.

Cross-sectional studies look at different groups of people at a specific point in time. The aim is to gather information about a particular topic by collecting data from a diverse group of individuals all at once. Then researchers analyze the data and try to find patterns or relationships, but they cannot establish what causes what.

The Frontiers of Psychology study also stressed that it's possible that both cannabis use and mental health problems may be caused by something else, like teens' susceptibility to stress and anxiety mentioned earlier.

To figure out whether cannabis causes mental health issues in teenagers, more research is needed.

Edited by: Carla Bleiker