Is there an effect of cannabis abuse on brain structure? Evidence is mixed. While there are certainly correlations, it is hard to conclude that it is a causal relationship. While longitudinal evidence supports a causal effect, data from twin and genetic studies show the same genes that are associated with cannabis use are also associated with brain structure (and probably brain development). So we cannot rule out causes besides cannabis use (e.g., behavioral traits associated with a greater likelihood of using cannabis are associated with smaller brain structure). Some recent interesting work has also shown that associations with cannabis are in fact attributable to polysubstance use - not cannabis alone.

If there are changes caused by cannabis, are they permanent? As opposed to alcohol, there have been very few studies of cannabis abstinence, though those that exist show some promise that any effects of cannabis on brain structure are probably reversible (provided use is stopped early enough).