Filled Under: marijuanna


4/15/2014 | For immediate release

Preliminary study suggests effects of drug even in those who are not addicted

Washington, DC — The size and shape of two brain regions involved in emotion and motivation may differ in young adults who smoke marijuana at least once a week, according to a study published April 16 in The Journal of Neuroscience. The findings suggest that recreational marijuana use may lead to previously unidentified brain changes, and highlight the importance of research aimed at understanding the long-term effects of low to moderate marijuana use on the brain.

Marijuana is the most commonly used illicit drug in the United States, with an estimated 18.9 million people reporting recent use, according to the most current analysis of the National Survey on Drug Use and Mental Health. Marijuana use is often associated with motivation, attention, learning, and memory impairments. Previous studies exposing animals to tetrahydrocannabinol (THC) — the main psychoactive component of marijuana — show that repeated exposure to the drug causes structural changes in brain regions involved with these functions. However, less is known about how low to moderate marijuana use affects brain structure in people, particularly in teens and young adults.

In the current 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 those with little to no history of marijuana use. Although psychiatric evaluations ruled out the possibility that the marijuana users were dependent on the drug, imaging data revealed they had significant brain differences. The nucleus accumbens — a brain region known to be involved in reward processing — was larger and altered in its shape and structure in the marijuana users compared to non-users.

“This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy,” said Carl Lupica, PhD, who studies drug addiction at the National Institute on Drug Abuse, and was not involved with this study. “These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users.”

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 emotion — 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 scientists found that the more the marijuana users reported consuming, the greater the abnormalities in the nucleus accumbens and amygdala. The shape and density of both of these regions also differed between marijuana users and non-users.

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” Breiter said.

This research was funded by the National Institute on Drug Abuse, the Office of National Drug Control Policy, Counterdrug Technology Assessment Center, and the National Institute of Neurological Disorders and Stroke.

The Journal of Neuroscience is published by the Society for Neuroscience, an organization of nearly 40,000 basic scientists and clinicians who study the brain and nervous system. Gilman can be reached at, Blood at, and Breiter at More information on marijuana and addiction can be found on

Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users


Marijuana is the most commonly used illicit drug in the United States, but little is known about its effects on the human brain, particularly on reward/aversion regions implicated in addiction, such as the nucleus accumbens and amygdala. Animal studies show structural changes in brain regions such as the nucleus accumbens after exposure to Δ9-tetrahydrocannabinol, but less is known about cannabis use and brain morphometry in these regions in humans. We collected high-resolution MRI scans on young adult recreational marijuana users and nonusing controls and conducted three independent analyses of morphometry in these structures: (1) gray matter density using voxel-based morphometry, (2) volume (total brain and regional volumes), and (3) shape (surface morphometry). Gray matter density analyses revealed greater gray matter density in marijuana users than in control participants in the left nucleus accumbens extending to subcallosal cortex, hypothalamus, sublenticular extended amygdala, and left amygdala, even after controlling for age, sex, alcohol use, and cigarette smoking. Trend-level effects were observed for a volume increase in the left nucleus accumbens only. Significant shape differences were detected in the left nucleus accumbens and right amygdala. The left nucleus accumbens showed salient exposure-dependent alterations across all three measures and an altered multimodal relationship across measures in the marijuana group. These data 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 dendritic arborization.


Correlative Measures of Cannabis Use

Ryan M. Smith, Research Assistant Professor

The Ohio State University

I read with great interest this article correlating neuroanatomical changes in casual cannabis users versus non-users. However, the authors repeatedly and explicitly state a causative relationship between the cannabis use and the anatomic changes throughout the article, only to state in the second-to-last paragraph that no causative relationship can be concluded due to the cross-sectional design of this study. Even more, the press release from the Society for Neuroscience [1] and the narrative in news stories resulting from this manuscript [2,3] do not explicitly state this very obvious caveat. While the correlative relationship reported here is statistically strong, a longitudinal study design is necessary to make the causative claims throughout the first 29 paragraphs and abstract of this manuscript. Again, this very critical distinction in study design is not noted until the end of the manuscript. A plausible alternative interpretation of this data is that the neuroanatomical abnormalities predate the drug use and make the individuals more likely to use cannabis. By reversing the causative relationship here, we would suppose that greater anatomical changes would cause increased consumption. With that said, there is abundant evidence showing an empirical link between tetrahydrocannabinol administration and neuroanatomical changes in rodents. However, the repeated claims of a causative relationship in this manuscript and related press articles are not warranted based solely on the results reported here.


[1]. Society for Neuroscience Press Release. “BRAIN CHANGES ARE ASSOCIATED WITH CASUAL MARIJUANA USE IN YOUNG ADULTS”. Accessed April 16th, 2014 at Archives/2014/Brain-Changes-Are-Associated-with-Casual-Marijuana-Use-in- Young-Adults

[2]. Ritter, M. “Study finds signs of brain changes in pot smokers”. Associated Press. Access April 16th, 2014 at -04-15-18-07-53

[3]. Dobuzinskis, A. “Casual pot use causes brain abnormalities in the young: study”. Reuters. Accessed April 16th, 2014 at idUSBREA3F04F20140416″