As cannabis becomes increasingly legalized and normalized, many users and policymakers are eager to understand the substance’s long-term effects—particularly on the brain. While some studies have downplayed structural brain changes in cannabis users, a 2014 study by Filbey et al. provides some of the most compelling evidence yet that chronic marijuana use, especially when begun at a young age, may lead to subtle but significant changes in both brain structure and connectivity.
“Chronic marijuana use is associated with complex neuroadaptive processes, and onset and duration of use have unique effects on these processes.”
Filbey et al., 2014
Using a combination of structural MRI, functional connectivity MRI, and diffusion tensor imaging (DTI), the study found that chronic cannabis users exhibited reduced gray matter volume in the orbitofrontal cortex (OFC)—a region crucial for decision-making and reward processing—and changes in how this region connects to others in the brain. The findings suggest that cannabis use may prompt complex neuroadaptive processes, with brain alterations shaped by both age of onset and duration of use. 
A Multi-Modal Look at the Brain
The study recruited 110 participants—48 chronic marijuana users and 62 age- and sex-matched nonusers—and used three different imaging techniques to get a comprehensive view of the brain:
• Structural MRI revealed reductions in gray matter volume in the bilateral OFC of cannabis users.
• Functional connectivity MRI (fcMRI) showed that cannabis users had higher connectivity within the OFC network, especially those who began using earlier in life.
• Diffusion tensor imaging (DTI) found higher white matter integrity (measured as fractional anisotropy) in the forceps minor tract—suggesting complex changes in the structural wiring of the brain.
Together, these findings point to a coordinated shift in both the anatomy and functional organization of the cannabis-using brain.
Reduced Gray Matter in the Orbitofrontal Cortex
The most prominent finding was a consistent reduction in gray matter volume in the orbitofrontal cortex. This region is rich in cannabinoid receptors and plays a key role in decision-making, impulse control, and evaluating rewards and punishments. Lower volume in this area may have implications for behavior, especially as it relates to addiction.
Interestingly, these changes were found even in participants who used cannabis exclusively—without comorbid tobacco or alcohol use—indicating that the observed brain differences were likely attributable to marijuana use itself.
Altered Brain Connectivity: Functional and Structural
Despite reductions in OFC volume, cannabis users showed increased functional connectivity within the OFC network. This could suggest a compensatory response—whereby the brain adapts to volume loss by reinforcing connections to maintain function. The increased connectivity was especially pronounced in users who began using cannabis at an earlier age.
On the structural side, cannabis users also showed higher integrity of the white matter tract (forceps minor) connecting the OFC. These changes were non-linear: DTI measures suggested initial strengthening of connections followed by decline with prolonged use—a potential reflection of dynamic neuroadaptive processes.
Age of Onset and Duration of Use Matter
The study found that age of onset was a major factor. The earlier a person began using cannabis, the more pronounced the increase in OFC connectivity. Conversely, longer duration of use was associated with declining structural integrity in white matter. These findings suggest that the brain’s response to cannabis is not fixed, but evolves over time—and that younger users may be more susceptible to long-term neuroplastic changes.
No Direct Link to IQ, but Effects Still Significant
While some other studies have reported cognitive decline among cannabis users, this study did not find a direct relationship between the observed brain changes and IQ scores. This suggests that while the changes may not translate immediately into measurable intelligence differences, they could still influence behavior and cognition in more nuanced ways—particularly those related to decision-making and self-control.
A Call for Caution and Further Research
The study doesn’t claim that cannabis causes brain damage—but it does show evidence of altered neuroanatomy and connectivity associated with chronic use. Importantly, these findings were observed in otherwise healthy individuals without psychiatric conditions or other drug dependencies.
As the researchers point out, longitudinal studies are needed to determine whether these changes are reversible with abstinence—or if they represent permanent adaptations. For now, the data suggest that heavy cannabis use, especially beginning in adolescence or early adulthood, can reshape the brain in subtle but measurable ways.
