By Nina Eng

On November 4th, about 64% of DC residents voted in favor of the DC Council Initiative 71, which would legalize “possession of minimal amounts of marijuana for personal use” for those who are 21 and older (DC Cannabis Campaign, 2014).

As it becomes an increasingly popular topic in the news, you may be wondering what sort of health risks are associated with marijuana smoking. Unfortunately, there really isn’t much information regarding the effects of marijuana on the brain and/or cognition, and the research that we do have is often conflicting. With more and more states opting to ‘legalize,’ it’s concerning to me that it still hasn’t become a priority in the science realm.

Marijuana is the most commonly used illicit drug in the US, with an estimated 100 million Americans having used it at least once (National Organization for the Reform of Marijuana Laws [NORML], 2013). It has also been adapted for medicine, generally for pain relief and appetite stimulation (NORML, 2013).

In 2004 Eldreth et al. found that regular smokers experienced decreased activation of the left perigenual anterior cingulated cortex (related to emotional sensations) and of the left dorsolateral prefrontal cortex (related to attention and working memory). However, no differences were found between the smoking and control groups when completing cognitive activities, such as the Stroops task, a psychological test used to observe reaction time and directed attention (De Young, 2014).

A follow-up 2005 study cited higher clusters of gray matter density around the pre-central gyrus in smokers compared to control groups; however, no conclusions could be made attributing these changes to marijuana smoking or any changes in behavior (Matochik et al., 2005).

Filbey et al. (2013) recently published a study reporting an association between marijuana usage and a decreased IQ, though Rogeberg et al. (2013) published a follow-up report 6 months later pointing out serious methodology flaws in the original study.

As you can see, much of the research has been extremely inconsistent and the consequences of those changes are still unknown.

As marijuana becomes more and more accessible, the amount of research conducted, especially longitudinal studies, must simultaneously increase if we are expected to make informed decisions regarding the future of our generation and many more to come.



 DC Cannabis Campaign. (2014). Ballot Initiative. Retrieved from:

De Young, R. (2014). Using the Stroop effect to test our capacity to direct attention: a tool for navigating urgent transitions. Retrieved from:

Eldreth, D.A., Matochik, J.A., Cadet, J.L., Bolla, K.I. (2004). Abnormal brain activity in prefrontal brain regions in abstinent marijuana users. NeuroImage, 23(3), 914-920. doi:10.1016/j.neuroimage.2004.07.032

Filbey, F.M., Aslan, S., Calhoun, V.D., Spence, J.S., Damaraju, E., Caprihan, A., Segall, J. (2013). Long-term effects of marijuana use on the brain. Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1415297111

Matochik, J.A., Eldreth, D.A., Cadet, J.L., Bolla, K.I. (2005). Altered brain tissue composition in heavy marijuana users. Drug and Alcohol Dependence, 77(1), 23-30. doi:10.1016/j.drugalcdep.2004.06.011

National Organization for the Reform of Marijuana Laws. (2014). About Marijuana. Retrieved from:

Rogeberg, O. (2013). Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status.Proceedings of the National Academy of Sciences110(11), 4251-4254.

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