Does long-term cannabis use affect brain structure?

The strongest evidence: Heavy use lowers brain activation during thinking tasks

The most direct evidence comes from a 2025 study of 1,003 young adults (ages 22-36) that used functional MRI to measure brain activity during seven different mental tasks. Heavy lifetime cannabis users — defined as those with over 1,000 lifetime uses — showed significantly lower brain activation during a working memory task compared to non-users [2]. The effect was moderate in size (Cohen's d = -0.28), meaning it was noticeable but not overwhelming, and it was specifically seen in brain regions critical for memory and attention: the anterior insula, medial prefrontal cortex, and dorsolateral prefrontal cortex [2]. Importantly, this association held even after accounting for recent cannabis use, suggesting the effect may be long-lasting.

A separate 2023 study of 73 cannabis users and 73 matched controls found that cannabis users had altered structural and functional connectivity in the insular and frontal opercular cortex, as well as the lateral and medial temporal cortex — areas involved in emotion, memory, and decision-making [5]. These changes were linked to the number of times cannabis was used, further supporting a dose-response relationship [5].

What about brain structure itself? The picture is more complex

While the evidence for functional changes (how the brain works) is fairly clear, the evidence for changes in brain structure (the actual volume or shape of brain regions) is more mixed. The 2023 connectivity study found that the overall organization of brain networks — the 'rich-club' hubs that coordinate communication — remained intact in cannabis users, even though local connections were altered [5]. This suggests that while cannabis may tweak specific circuits, it doesn't necessarily cause widespread structural damage.

However, a large 2023 study of nearly 12,000 children (ages 9-11) found that prenatal cannabis exposure was linked to a gradual decrease in intracranial volume (total brain size) and visuo-perceptual processing ability as the children aged [3]. This effect became more pronounced over time, suggesting that early exposure can have a lasting impact on brain development. It's important to note that this is a different population (developing brains) than adult users, so it doesn't directly answer the question about adult use, but it does show that cannabis can alter brain structure under certain conditions.

The gap between best-case and typical use: Not all users are affected equally

A key takeaway from these studies is that the effects depend heavily on the pattern of use. The 2025 study found that only heavy lifetime users (over 1,000 uses) showed significant brain activation differences; moderate users (10-999 uses) did not differ from non-users [2]. This suggests a threshold effect: occasional or light use may not produce the same brain changes as heavy, sustained use.

Additionally, the 2023 connectivity study found that while local brain networks were altered, the global network organization remained normal [5]. This means that the brain can compensate for some local changes, and the overall 'wiring diagram' may stay intact. So, while heavy use is linked to real changes, the brain may be resilient to moderate use — at least in the short to medium term.

About These Sources

This answer is built on 5 peer-reviewed studies — published from 2023 to 2025, 2 from 2024 or later, 5 in Q1 journals, collectively cited 85 times — selected as the most relevant from 5 studies that passed quality screening, drawn from 51 papers retrieved from a database of over 500 million.

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