Adding to earlier evidence that marijuana may be linked to lasting neurological changes, a new study in the Journal of Neuroscience today finds that even casual pot smoking may have an effect on the size and structure of certain brain regions. The new research reports that for each additional joint a person smokes per week, the greater the odds of structural changes to areas involved in motivation, reward, and emotion. Though it seems like the country has embraced pot as a relatively harmless option in recent years, the authors of the study say that their findings suggest otherwise, especially for young people whose brains are still developing.
"This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences," said study author Hans Breiter, psychiatry and behavioral sciences professor at Northwestern University Feinberg School of Medicine and psychiatrist at Northwestern Memorial Hospital. "Some of these people only used marijuana to get high once or twice a week. People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case."
In the new study, the team looked at the brains of people 18-25 years old, some of whom smoked pot recreationally and some who did not. None of the participants showed any signs of being addicted to the drug.
Using different brain imaging techniques, the researchers were able to measure the volume, shape, and grey matter density of two key structures: the nucleus accumbens and the amygdala. The nucleus accumbens is involved in the reward circuit, including pleasure-seeking and motivation, and it’s strongly linked to addiction. The amygdala is involved in emotion, particularly in fear, anxiety, and the stress response, and in drug craving.
The team found that both brain structures varied in multiple ways, according to the number of joints per week the participants smoked – in other words, the more joints smoked, the more brain changes were evident. The nucleus accumbens was especially likely to show alterations in shape and density, and to be larger, as a function of joints per week.
"These are core, fundamental structures of the brain," said study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General Hospital and psychiatry professor at Harvard Medical School. "They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."
What’s interesting about the study is that it suggests that even sometimes-smokers show changes in the brain. What’s not clear is whether there were differences in the pot smokers’ behavior or cognitive function. But the authors suggest that the brain changes seen here may be a sort of precursor to addiction: Earlier studies in animals have shown that the active ingredient in pot, tetrahydrocannabinol (THC), may affect neural connectivity, which could be an early sign of a bourgeoning addiction.
"It may be that we're seeing a type of drug learning in the brain," said author Jodi Gilman, at Massachusetts General Center for Addiction Medicine. "We think when people are in the process of becoming addicted, their brains form these new connections.”
Although a majority of people in the country support legalization of marijuana, not everyone is so convinced. Last year, Breiter’s team showed that everyday pot smoking in teenagers was, even two years after stopping, linked to brain abnormalities and to poorer working memory. "With the findings of these two papers," Breiter said, "I've developed a severe worry about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain."
The study will no doubt attract a lot of debate, as it raises as many questions as it answers. More research is clearly needed to know just how pot affects the brain and behavior over the long term. In the meantime, you may just want to pass on the pot – or at least wait till you're 30 and your brain is done developing before you use it as a test subject in your own experiment.
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