“No statistically significant differences were found between daily users and nonusers on volume or shape in the regions of interest.
Effect sizes suggest that the failure to find differences was not due to a lack of statistical power, but rather was due to the lack of even a modest effect.
In sum, the results indicate that, when carefully controlling for alcohol use, gender, age, and other variables, there is no association between marijuana use and standard volumetric or shape measurements of subcortical structures.
The press may not cite studies that do not find sensational effects, but these studies are still extremely important. While the literature clearly supports a deleterious short-term effect of marijuana on learning and memory, it seems unlikely that marijuana use has the same level of long-term deleterious effects on brain morphology as other drugs like alcohol.”