Can Cannabis Alter The Teenage Brain?

By Lindsey Weedston 01/23/19

A recent study examined how marijuana use impacts the gray matter in the teenage brain.

a teen reaching for a joint

A recent study consisting of brain scans of 46 teens in Europe found that smoking just one or two joints seemed to produce changes in the gray matter of their brains.

The teens appeared to have a greater amount of this tissue, which is a major component of the central nervous system and is responsible for information processing. However, this does not necessarily mean that more gray matter is better.

According to a 2006 article in the Postgraduate Medical Journal, the brain naturally shrinks as people age. This process is called “pruning” and is part of the normal development process at all ages.

This new study, titled “Grey Matter Volume Differences Associated with Extremely Low Levels of Cannabis Use in Adolescence,” notes that gray matter volume (GMV) in the temporal regions of the brain is “associated with contemporaneous performance on the Perceptual Reasoning Index and with future generalized anxiety symptoms in the cannabis users.”

While there has been little research on the effects of cannabis on the brain compared to substances like alcohol, it is generally considered true that permanent changes and damage to the young, developing brain are more significant due to the compounding issues that development disruption causes over time. However, it’s difficult to determine whether the increased gray matter observed in the studied teens is a bad thing, a good thing or a little of both.

“At the age at which we studied these kids (age 14), cortical regions are going through a process of thinning," said Hugh Garavan, lead author of the study and a professor of psychiatry at the University of Vermont School of Medicine, to NBC News. "So, one possibility is that the cannabis use has disrupted this pruning process, resulting in larger volumes (i.e., a disruption of typical maturation) in the cannabis users. Another possibility is that the cannabis use has led to a growth in neurons and in the connections between them."

Gray matter can also be altered by a number of common activities other than drug use. Studies have found that meditation can result in changes to this part of the brain. Others have found that habitual interaction with action video games reduces gray matter in the hippocampus while playing 3D platformer video games increases it. Even becoming pregnant has shown to create significant changes in gray matter structure that last for two years after birth.

In this latest study, the 46 teens self-reported smoking very small amounts of cannabis in their lifetimes, equivalent to one or two joints, and reported that they had not consumed any other illicit substances.

Not only did the study find greater GMV levels around the amygdala, hippocampus and other areas of the brain, follow-ups found higher levels of “sensation seeking” and anxiety symptoms among the cannabis-using teens compared to controls. However, the authors of the study specifically stated that these behavioral differences were unrelated to the amount of gray matter.

“Of the behavioral variables tested, only sensation seeking and agoraphobia differed between the cannabis users and controls and these factors were not related to GMV differences,” the study reads.

The authors also noted that behavioral differences should be “interpreted with caution” due to the low sample size, but they are notable as “panic and anxiety symptoms are frequently reported side effects by naïve and occasional cannabis users.”

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Lindsey Weedston is a Seattle area writer focused on mental health and addiction, politics, human rights, and various social issues. Her work has appeared in The Establishment, Ravishly, ThinkProgress, Little Things, Yes! Magazine, and others. You can find her daily writings at Twitter: