The Developing Brain

The brain is the most complex organ in the human body. To say that there is still much to learn about the development and functioning of the brain would be an understatement. However, it is in this area of the unknown that scientists thrive and research on the brain continues to offer new and exciting findings and theories. Within this research, there is one area which is beginning to get more notice by the scientific community – that is, the effects of marijuana on the brain and more specifically, on the developing teenage brain.

As with all research of this nature, conclusions are presented as probable risks and not fact. However, based on the studies to date, there is enough evidence to suggest that it is vital that young people be informed of the risks that consuming marijuana presents to their mental health.

“The teenage brain is not just an adult brain with fewer miles on it. It is a paradoxical time of development. These are people with very sharp brains, but they’re not quite sure what to do with them.” – Frances E. Jensen, Professor of Neurology, Children’s Hospital Boston and Harvard Medical School. In the video below she is translating the most up-to-date research on the teen brain which she shares with parents, teachers and teens during her presentation, “Teen Brain 101″.


In 2010 David Suzuki’s “The Nature of Things” aired an episode on CBC-TV titled “The Downside of High”. This educational documentary dealt with THC, how it is produced, and how today’s marijuana has stronger effects on the brain. It particularly explores the link to mental health challenges such as schizophrenia.

View the Program


Depression and Anxiety

This is a relatively new area of research and there is much debate about the links between marijuana use and depression. Much of the research to date has not been able to establish a direct link between marijuana use and depression but rather conclude that the factors (genetic, environmental, and social) that trigger depression can also lead to marijuana use. Of concern however is that more recent studies in this area have shown that heavy marijuana use and depression accompany each other more than one might expect leading some researchers to investigate a more causal link between marijuana use and depression, especially in adolescents.

According to a recent study by Dr. Gabriella Gobbi from the Research Institute of the McGill University Health Centre, daily consumption of marijuana in teens can cause depression and anxiety and have an irreversible long-term effect on the brain. According to her findings, there is an apparent action of marijuana on two important compounds in the brain, serotonin and norepinephrine, which are involved in the regulation of neurological functions such as mood control and anxiety. Dr. Gobbi cites that teenagers who are exposed to marijuana have decreased serotonin transmission, which leads to mood disorders, and increased norepinephrine transmission, which leads to greater long-term susceptibility to stress. This is one of the first studies to demonstrate that marijuana consumption causes more serious damage during adolescence than adulthood.

Certainly more research is needed to better understand these possible connections.


Cannabis and adolescence: A dangerous cocktail


Cognitive Functioning

From research cited by the Canadian Centre on Substance Abuse (CCSA), there is evidence that those who begin using marijuana at an early age, when the brain is still developing, may be more vulnerable to lasting neuropsychological deficits than those who begin use later in life. Such deficits are often not severe or grossly debilitating, but rather more subtle in nature. According to CCSA, visual scanning is a cognitive function that undergoes a major maturational process around 12-15 yrs of age. Visual scanning is the ability to quickly and efficiently find relevant information in our surroundings. Research has shown that early on-set marijuana users (before age 16), exhibited decreased visual scanning skills. Lack of this ability reduces the speed and accuracy of our reactions and responses. This can affect everything from driving to academic achievement.

The Brain

One study quoted by the CCSA reported that long term marijuana users who began using before the age of 17, had smaller brains, with a lower percentage of gray matter, the processing component of our brain.

Mental Health

According to CCSA, psychotic disorders involve disturbances in the dopamine neurotransmitter systems. Cannabinoids, such as THC found in marijuana, are associated with increased release of dopamine. This may be why frequent marijuana users are found to be at an increased risk of experiencing a psychotic outcome. The relationship between marijuana use and psychosis appears to be stronger in people who show a predisposition to psychosis. There is still much debate as to whether the age of first use affects the risk of developing psychosis. Some studies have observed a correlation while others have not. As per CCSA, the effects of marijuana may be greater in those who begin use early in adolescence because their developing brains are vulnerable to persistent alterations that affect behavior. It may also be that adolescent users are at greater risk because they tend to smoke marijuana more frequently.


CCSA – Chronic Use and Cognitive Functioning and Mental Health


Rational Thought

As stated elsewhere throughout this site, the part of the brain that controls emotions develops faster in an adolescent than the part of the brain that controls rational thought. According to a presentation by Dr. Jean Clinton from the Offord Centre for Child Studies, in which she cites Dr. Ron Dahl, a pediatrician and child psychiatric researcher at the University of Pittsburgh Medical Center – “In calm situations, teenagers can rationalize almost as well as adults. But stress can hijack hot cognition and decision-making. The frontal lobes help put the brakes on a desire for thrills and taking risk — a building block of adolescence; but, they’re also one of the last areas of the brain to develop fully”. Understanding this is imperative because it means young people are more likely to take risks, such as experimenting with substance use without fully thinking about the potential outcomes of such decisions.


Offord Centre – Dr, Jean Clinton PowerPoint


Scientists continue to learn a great deal about how the active ingredient in marijuana – THC – acts in the brain. When a person smokes marijuana, THC rapidly passes from the lungs into the bloodstream and the chemical is carried to organs throughout the body, including the brain. In the brain THC connects to specific sites called cannabinoid receptors on nerve cells and thereby influences the activity of those cells. Some areas of the brain have many cannabinoid receptors while others have few or none. It is significant that many cannabinoid receptors are found in the parts of the brain that influence pleasure, memory, thought, concentration, sensory and time perception, and coordinated movement. These areas of the brain with an abundance of cannabinoid receptors are of particular interest to scientists that study the impact of THC on the developing brain.

Read more about THC



A Parents Guide to the Teen Brain

National Institute of Mental Health - The Teen Brain: Still Under Construction

Mentor Foundation – Adolescent Brain Development and Drug Abuse