A study investigating recreational marijuana use in young adults has found that users had differences in two areas of the brain, the amygdala and the nucleus accumbens, when compared with non-users. The study has been published in The Journal of Neuroscience.

Cannabis, or marijuana, is the most commonly used illicit drug in the United States, but relatively few studies have investigated how it affects the brain. Although some animal studies have demonstrated changes in regions of the brain involved in learning and memory, attention and decision-making after exposure to the main psychoactive component of cannabis (THC), only a handful of similar studies exist in humans. The few studies that have been carried out also show contrasting results, which may be due to differences in methodologies. They also usually focus on heavy users as oppose to occasional users.

In this study, scientists used high-resolution MRI to scan the brains of 20 marijuana users and 20 non-users, between the ages of 18-25. The scans were used to collect data on gray matter density, brain volume and morphology. They matched the participants on age, sex, race and years of education. The marijuana users reported that they used the drug at least once a week, but were not dependent. The non-users had never used it more than 5 times in their lifetime. They also corrected for things like alcohol consumption as the users reported that they drank more alcohol than the non-users.

The researchers found that the marijuana users, compared with the non-users, had structural abnormalities in gray matter density, volume and shape of two areas of the brain; the nucleus accumbens and amygdala. These regions have been implicated in reward and aversion and therefore likely play a role in addiction. They are also involved in the euphoria associated with the rewarding effects of drugs.

The researchers also found that some of the abnormalities were exposure dependent. Finally, the researchers also suggested that cannabis use may be associated with a disruption of neural organization in the nucleus accumbens and amygdala, but this is speculation.

These data therefore suggest that recreational marijuana use in young adults may lead to alterations in the core reward structures. It's important to remember, however, that correlation does not imply causation- the study can infer an association but not a definitive cause. The number of participants was also small and the study did not investigate whether cannabis use affected cognition as well as brain morphology. Therefore although this particular study is important, it also lays the foundations for further in-depth research into this interesting area.