How Early Marijuana Use Affects Brain Development Becomes Clearer

The age at which an adolescent begins using marijuana may affect typical brain development, according to researchers at the Center for BrainHealth in the School of Behavioral and Brain Sciences at UT Dallas.

In a paper recently published in Developmental Cognitive Neuroscience, scientists describe how marijuana use and the age at which the use starts, may adversely alter brain structures that govern higher order thinking.

In the study, participants who began using marijuana at the age of 16 or earlier showed indications of arrested brain development in the prefrontal cortex, which is responsible for judgment, reasoning and complex thinking. Individuals whose marijuana use started after age 16 demonstrated the opposite effect with signs of accelerated brain aging.

“Science has shown us that changes in the brain occurring during adolescence are complex. Our findings suggest that the timing of cannabis use can result in very disparate patterns of effects,” said Dr. Francesca Filbey, the study's principal investigator and director of cognitive neuroscience of addictive behaviors at the Center for BrainHealth. “Not only did age of use impact the brain changes but the amount of cannabis used also influenced the extent of altered brain maturation.”

For the study, researchers analyzed MRI scans of 42 heavy marijuana users, ages 21 to 50, whose use began during adolescence and continued throughout adulthood, at least once a week. Twenty participants were categorized as early onset users with an average beginning consumption age of 13.18, and 22 were late onset users with an average of 16.9. 

In typical adolescent brain development, the brain prunes neurons, resulting in reduced cortical thickness and greater gray and white matter contrast, according to Filbey. Typical pruning also leads to increased gyrification, the addition of wrinkles or folds on the brain’s surface.

The study reveals that the more marijuana early onset users consumed, the greater their cortical thickness, the less gray and white matter contrast, and the less intricate the gyrification as compared to late onset users. For early onset users, the extent of brain alteration was directly proportionate to the amount of use and grams consumed. Late onset users showed brain changes that typically manifest later in life: thinner cortical thickness, stronger gray and white matter contrast.

“In the early onset group, we found that how many times an individual uses and the amount of marijuana used strongly relates to the degree to which brain development does not follow the normal pruning pattern. The effects observed were above and beyond effects related to alcohol use and age. These findings are in line with the current literature that suggest that cannabis use during adolescence can have long-term consequences,” Filbey said.

Filbey said that a longitudinal study would be necessary to establish a causal relationship between brain alterations and marijuana use. Her future studies will explore cognitive and behavioral changes associated with structural brain change. This study was funded by the National Institute on Drug Abuse.