Even Casual Marijuana Use Changes the Brain

While many are aware of the detrimental effects of heavy marijuana use on motivation, attention, learning, and memory, casual use is still generally considered fairly harmless. Yet a new study in the Journal of Neuroscience suggests even recreational use alters the brain.

Researchers found changes in areas of the brain involved in emotion and motivation in young adults who smoked marijuana at least once a week. “Currently, we don’t know how much marijuana, if any, is safe, and this study indicates that there are observable differences in brain structure with marijuana, even in recreational young adult users,” says study author Jodi Gilman, PhD, of the Center for Addiction Medicine at Massachusetts General Hospital in Boston.
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“This study shows that we should be cautious about marijuana, and discourage use in adolescents, whose developing brains may be even more susceptible to cannabis-induced changes,” she says.

Gilman and colleagues used magnetic resonance imaging to analyze the brains of 40 subjects, ages 18 to 25, who smoked marijuana at least once per week. None of them met the criteria for marijuana dependence, which includes the development of physiological and psychological tolerance; use of the drug even in the presence of adverse effects; and the giving up of social, occupational, or recreational activities because of marijuana use. 

“When we compared brain scans of people who used marijuana with scans from non-users, we found differences in fundamental regions of the brain,” Gilman says. “These findings in recreational users indicate that abnormalities can be detected early on, before the development of dependence.” 

In the brains of the marijuana users, the nucleus accumbens—a brain region involved in reward processing—was not only larger but also had an altered shape and structure. And the more marijuana the smokers used, the greater the abnormalities in both this region and the amygdala, a part of the brain involved in emotion.

“The regions affected are very important for a range of behaviors—from reward processing to motivation to emotional processing,” Gilman says. “The next important thing to investigate is how these structural abnormalities relate to functional outcomes. For example, how changes in the shape of the nucleus accumbens relate to behaviors such as memory, reward sensitivity, and motivation.”

She says they also are interested in examining how different exposure to marijuana (i.e. different concentrations of THC, the psychoactive compound in marijuana) affects the brain, as THC content is higher now than it has been in the past.

—Colleen Mullarkey

Reference

Gilman JM, Kuster JK, Lee S, Lee MJ, Kim BW, Makris N, et al. Cannabis use is quantitatively associated with nucleus accumbens and amygdala abnormalities in young adult recreational users. J Neurosci. 2014 Apr 16;34(16):5529-38.