Several recent studies have suggested that smoking marijuana is associated with physical changes in certain regions of the brain, both in terms of shape and volume, although they could not establish cause and effect. Now, new research which set out to replicate these investigations using a more robust experimental design has produced conflicting results. According to the study, daily marijuana use, in both adults and adolescents, is not associated with any significant differences in either the shape or volume of the regions investigated. The work has been published in The Journal of Neuroscience.

Given the current changing trends towards the acceptance and use of marijuana, it’s important to thoroughly investigate the possible risks associated with the drug so that decisions regarding legalization and classification can be based on scientific evidence. Numerous studies have therefore looked into the potential effects of marijuana use, and several have concluded that smoking marijuana is associated with changes in the brain. One investigation even concluded that frequent marijuana use was associated with cognitive decline and a decrease in IQ. However, results have not been consistent throughout different studies.

Some investigations, for example, found that marijuana use is linked to a decrease in the size of certain areas of the brain, whereas others concluded that marijuana is associated with an increase in volume of the same areas. While the studies were interesting, it’s important to note that, because of the way they were designed, it was not possible to establish cause and effect. Furthermore, the studies may not have adequately controlled for alcohol use, which is a particularly important issue given that it is well established that alcohol abuse can have a detrimental effect on brain structure as well as volume and mental ability.

In order to address this issue and hopefully provide some clarity, scientists designed a well-controlled study that set out to investigate the potential effects of daily marijuana use on both adults and adolescents. In particular, they wanted to compare the brains of users and non-users by examining the morphology of numerous different regions which were the focus of previous studies: the nucleus accumbens, amygdala, hippocampus and cerebellum.

For the study, 29 adult daily marijuana users were enrolled, alongside 29 adult non-users. A group of 50 adolescent daily users were also recruited, once again alongside a sample of 50 adolescent non-users. Importantly, the researchers closely matched the groups on many possible confounding variables, such as depression, age, tobacco use and gender. Furthermore, they were matched on alcohol use to a much greater extent than previous studies.

After carrying out MRI scans on the participants and conducting statistical analyses, the researchers failed to find any differences in the volumes of any of the brain regions investigated. According to the researchers, this could suggest that previously observed differences might be due to inadequate control for alcohol use, given the fact that even modest alcohol abuse has been linked to changes in the brain.

Although this study was more robust in terms of matching groups, it is necessary to note that it still has some important limitations. For example, it still cannot establish causality, and did not take into account socioeconomic factors or the history of marijuana use, such as when they began using the drug.

[Via The Journal of Neuroscience]