Tech + Health


No, Weed Won’t Rot Your Brain

Can casual marijuana use damage the brains of young adults? A new study says yes—but its participants suggest otherwise.

All across the Internet, headlines are screaming Buzzkill and Marijuana Makes Young Brains Go to Pot. But a new study, published in the Journal of Neuroscience, does not in any way prove that casual marijuana use is bad for your brain.

In order to understand why, all you need to do is actually read the research—and be able to think a bit critically. You don’t need to know anything particular about fMRI or any other scary acronyms and you don’t need to know your amygdala from your Shatner’s Bassoon. You don’t even need to know any statistics.

Here’s the first big problem. The 20 marijuana-smoking participants, who took the drug at least once a week, were deliberately selected to be healthy. If they had any marijuana-related problems—or any psychiatric problems or other issues—they were excluded from participating.

Are you beginning to see what’s wrong? Although the pot-smoking participants showed brain differences in comparison to the controls who were also selected to be normal—both groups were normal! If the smokers had any marijuana-related problems or any type of impairment, they would not have been included in the first place. Therefore, the brain changes that the researchers found were—by definition—not associated with any cognitive, emotional, or mental problems or differences.

“I’m disappointed that scientists are still able to publish high-profile papers that only look at neuroimaging without a behavioral endpoint,” says Carl Hart, an associate professor of psychology at Columbia University who was not associated with the research (Disclosure: he and I worked on a book project together). Hart compares the findings to brain differences found between the genders. “There are structural differences between men and women in certain areas,” he says, but they don’t predict differences in ability. “We don’t say this means women are impaired,” he adds.

The authors claim that the differences they saw could mean that these participants are at risk of future problems—but we know that 35 percent of young adults 18-20 have smoked marijuana in the past year, with a full 1 in 5 reporting smoking at least once in the past month.

“If the smokers had any marijuana-related problems or any type of impairment, they would not have been included in the first place.”

Once they reach age 26, however, less than 1 percent have marijuana problems serious enough to be classified as addiction. What that means is that whatever brain changes are seen in casual users, they don’t predict addiction, otherwise, all casual users would become addicted—or at least, a much larger proportion than actually do. We’ve already had several generations of American adults who survived far higher rates of marijuana use than we see now—without encountering a major epidemic of cognitive impairment, schizophrenia, or lack of motivation.

Sadly, this isn’t even the only issue with the study. “Just casual use appears to create changes in the brain in areas you don’t want to change,” lead author Hans Breiter, professor of psychiatry at Northwestern University Feinberg School of Medicine, told USA Today. But note that Breiter hedges by saying “appears to create,” rather than “creates.” That’s because this type of study cannot determine cause and effect: while it did show that the heavier users in the sample seemed to have more extreme changes than the lighter users, this does not prove that higher doses cause greater brain changes. That’s because pre-existing differences in people’s brains may lead them to use more or less marijuana— and the scans may simply be picking up on these differences.

Does this imply that marijuana is completely benign and everyone should smoke all day, every day? Of course not! But what it does mean is that, as we consider policy changes like legalization, we need a far more skeptical and intelligent press. Marijuana itself may or may not impair cognition— but discussions of marijuana policy clearly do so, in a way that is detrimental to our political health.