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Casual Marijuana Use Linked to Brain Changes in Young Adults

Light cannabis use by young adults might alter brain structure in two regions.

Marijuana is the most commonly used illicit drug in the United States. Recent studies have shown that heavy marijuana use by young adults is associated with changes in brain structure linked to a loss of motivation, increased anxiety, and cognitive impairments.

A study from April 2014 has found that even light use of marijuana—smoking pot once a week—may cause structural changes in the size and shape of two brain regions. This is the first study to show casual use of marijuana may be related to major brain changes.

The study titled “Cannabis Use is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users” was published April 16 in The Journal of Neuroscience. The researchers found a direct correlation between the quantity of cannabis use and the degree of changes to the developing brain of people under 25-years-old.

Marijuana Use Can Alter the Amygdala and Nucleus Accumbens

The more pot a young adult smokes the more the brain is altered. The findings suggest that marijuana exposure, even in young recreational users, is associated with exposure-dependent alterations of the neural matrix of core reward structures and is consistent with animal studies of changes in the gray matter volume of the amygdala and nucleus accumbens.

In the recent study, Jodi Gilman, PhD, Anne Blood, PhD, and Hans Breiter, MD, of Northwestern University and Massachusetts General Hospital/Harvard Medical School used magnetic resonance imaging (MRI) to compare the brains of 18- to 25-year olds who reported smoking marijuana at least once per week with people who had a minimal to zero history of marijuana use.

The team of scientists compared the size, shape, and density of the nucleus accumbens and the amygdala—a brain region that plays a central role in anxiety and emotional regulation—in 20 marijuana users and 20 non-users. Each marijuana user was asked to estimate their drug consumption over a three-month period, including the number of days they smoked and the amount of the drug consumed each day.

The MRI brain images showed significant brain differences even in light marijuana users. In particular, the nucleus accumbens—which is a brain region linked to reward processing and motivation—was larger and altered in its shape and structure in the marijuana users compared to non-users. Of particular interest to the researchers was that the nucleus accumbens was abnormally large, and its alteration in size, shape and density was directly correlated to how many joints an individual had smoked.

The researchers were unable to assess the THC content of the marijuana each person smoked because the study was retrospective. In general, THC levels are estimated to range from 5 to 9 percent or even higher in currently available marijuana. As a note of interest, the THC of marijuana during the 1960s and 1970s was generally around 1 to 3 percent.

Medical Marijuana Has Many Pros

In recent months, I have written a few Psychology Today blog posts based on studies linking heavy marijuana use to negative consequences in young adults. I have received a wide range of heartfelt personal emails from parents of children and individuals who have benefitted from the use of medical marijuana and attest that the pros to medicinal cannabis greatly outweigh the cons.

Clearly, cannabis has many beneficial medicinal purposes. Also, many adults I know use marijuana casually and don't appear to suffer any dramatic negative consequences. The human brain is still developing until age 25 which makes it particularly vulnerable to exogenous substances. Car rental companies base their decision not to rent vehicles to people under 25 based on neuroscience and the vulnerabilities of a human brain that is still developing.

Although I haven’t come across new research on the pros of medical marijuana, I wanted to balance my writing on the pros and cons of marijuana by including a link to 105 peer-reviewed "Medical Studies Involving the medical use of Cannabis and Cannabis Extracts between 1990 - 2012."

Conclusion: More Research on Light Marijuana Use Is Needed

The researchers conclude that this study challenges the idea that casual marijuana use doesn’t have negative consequences on brain development for young adults. Many young people assume that getting high occasionally won’t have any harmful effects. The researchers hope this study will make young adults think twice before smoking marijuana even once a week.

The researchers believe their data suggests even casual marijuana use causes brain changes but acknowledge that further research, including longitudinal studies, is needed.

If you'd like to read more on this topic, check out my Psychology Today blog posts:

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