The stereotype of an avid marijuana smoker is not flattering: slow, unmotivated, a little bit dulled by all that weed. But the science to back up this stereotype is far from clear.
Research is mixed as to whether marijuana causes declines in intelligence and functioning over time. Animal studies and some brain scans in humans provide reason for concern: Marijuana is psychoactive, and may cause structural brain changes. In people, weed's cognitive effects seem to last at least several weeks after use, long after the person stops feeling intoxicated. But only a few studies have revealed insight into whether pot lowers IQ in the long term, and those studies have returned conflicting results. [11 Odd Facts About Marijuana]
The recreational use of marijuana is now legal in four states (Alaska, Colorado, Oregon and Washington) and the District of Columbia. Many other states have decriminalized the drug, and some also allow the use of medical marijuana. And a 2013 Gallup poll found that 58 percent of Americans support marijuana legalization, up from a mere 12 percent in 1969. In other words, the drug has never been more mainstream.
Despite the loosened regulations, however, marijuana research has lagged. Much of the reason has to do with the difficulty of getting marijuana for study, said Nick Jackson, a statistician at the University of Southern California and a co-author of one of the few longitudinal studies (which follow people over time) on marijuana use. In fact, there has been about three times more animal research on cocaine than on marijuana.
"You didn't need to jump through the same number of hoops to get cocaine to test on your animals as you do to get marijuana," Jackson told Live Science. The National Institute on Drug Abuse and the Drug Enforcement Administration contract with only one lab (at the University of Mississippi) to make marijuana available to researchers.
The Food and Drug Administration recently relaxed its rules for approving marijuana research, Jackson said. "Things are changing slowly but surely," he told Live Science. "But our research in this area is far behind where it needs to be." [The Drug Talk: 7 New Tips for Today's Parents]
That's why the answer to the question, "Does pot make people stupid?" is more complicated than it might seem.
Animal studies suggest that pot is not necessarily great for the brain. Rats exposed to marijuana's active ingredient, tetrahydrocannabinol (THC), experience brain changes and cognitive impairment. And short-term studies with human subjects clearly point to impacts on memory, learning and attention even once a user has sobered up. One 1996 study published in the Journal of the American Medical Association, for example, found that daily marijuana users did more poorly on tests of attention and executive function (such as planning and self-control) than people who'd smoked pot only once the month before, even though both groups abstained for at least 19 hours before the testing. The drug's effects may persist at least 20 days after smoking, according to a 2011 review on the topic.
But the burning question is whether pot hurts the brain in the long run. Does smoking the occasional joint as a teenager mess up your cognitive abilities for life? What if you pick up a pot habit as an adult, after the brain has completed its adolescent growth spurt? Does the dose make a difference?
Here, the answers are a lot fuzzier. Brain-scan studies in humans suggest that pot may be linked to anatomical brain changes, such as shrinking of the amygdala, a brain region that processes emotion, reward and fear. In some people with genetic vulnerability, such brain changes might be enough to tip someone into schizophrenia, which is more common in people who have used marijuana. However, the genes in question may lead people to smoke more pot and to be more prone to schizophrenia, rather than directly causing the link between pot and psychosis.
And that's the problem with trying to tease out pot's effects: People who use the drug are likely different from people who don't. Thus, studies comparing smokers with nonsmokers at a moment in time are of limited use: Maybe pot caused the cognitive effects you might find, or maybe some other factor explains the difference. [7 Ways Alcohol Affects Your Health]
To truly tease out the effect of marijuana alone, researchers have to follow people over time, ideally gathering information about their cognition and intelligence before they began using pot. Only a handful of studies have done this so far.
The first, published in the journal Neurotoxicology and Teratology in 2005, found that being a current regular user of marijuana led to deficits in memory, IQ, processing speed and memory, but people who had used the drug in the past but had since stopped did not show long-term effects three months after quitting. However, that study followed 113 teenagers who used marijuana for an average of only two years.
A bigger, longer-term study, published in the journal Proceedings of the National Academy of Sciences in August 2012, did not bode well for pot connoisseurs. Researchers followed 1,037 New Zealanders from birth to age 38, assessing their cognitive function at age 13 (before any participants had started using cannabis) and again at age 38. Participants reported their cannabis use at age 18, 21, 26, 32 and 38, giving researchers an opportunity to determine whether cognitive effects differed depending on when a person started using marijuana and how long he or she continued to use it.
That study found global declines in cognition, including an average drop in IQ of about 6 points in people who had used marijuana. The biggest effects were seen in persistent users — people who reported having consumed marijuana in at least three interviews between the ages of 18 and 38. Notably, the deficits were not found in people who started using marijuana as adults, but were strong in people who took up the habit as teens. The researchers also had participants' close friends or family members fill out questionnaires on the participants' daily functioning, and found that those who had used marijuana were worse off than those who had not.
"Marijuana is not harmless, particularly for adolescents," study researcher Madeline Meier, now a psychology professor at Arizona State University, concluded in a statement sent to Live Science. [10 Facts Every Parent Should Know About Their Teen's Brain]
Not all of the longitudinal data agrees, however. For a study published in the Journal of Psychopharmacology in January 2016, researchers followed 2,235 British teenagers, about a quarter of whom had tried pot at least once by age 15. The researchers found no link between cumulative marijuana exposure at age 15 and IQ or educational performance at age 16.
The study was based on a short time frame, but even longer-lasting investigations returned conflicting results. In February 2016, researchers published the results of a study following marijuana users and nonusers into middle age. They analyzed the verbal memory, processing speed and executive function (planning abilities and self-control) in 3,385 participants in the Coronary Artery Risk Development in Young Adults (CARDIA) study. About 84 percent (2,852) had used marijuana at some point, but only 11 percent (392) had used it in middle age. The study showed that after the researchers accounted for other factors that could have affected the results, such as other drug use and demographics, cumulative pot use was linked to worse verbal memory. For every five years of marijuana use, a person would remember one less word, on average, from a list of 15 they were asked to memorize. However, no declines in executive function or processing speed were found.
Turning to twins
Although all of these studies controlled for factors that might influence cognition — demographics, other drug use, education — those statistics aren't an exact science. Jackson, along with University of Minnesota Twin Cities researcher Joshua Isen, came up with a way to control the comparison.
The researchers were working with two data sets of more than 3,000 identical twins, meaning they had the same genetic makeup and the same home environment. The pairs of twins had undergone intelligence testing between the ages of 9 and 12 (before using marijuana), and between the ages of 17 and 20 (after some had started using the drug). By comparing marijuana users with their non-using twins, the researchers were able to control for the home and environmental factors that aren't necessarily captured in traditional statistical adjustments.
The analysis revealed that, overall, marijuana users were indeed cognitively worse off than nonusers in late adolescence. But the users were also worse off before they started using pot. And when researchers compared the pot users to their own non-using twins, they found that the sibling pairs ended up in the same place, cognitively speaking. Thus, it wasn't the pot use that was causing the differences between the group of pot users and non-users. It was some unexplored factor that affected both twins, whether they smoked pot or not.
"We believe that what we're looking at has something to do with the common environment that these twins share, something about their family environment or peer environment or school environment," Jackson said.
That does not mean that marijuana is harmless, Jackson said. Animal studies do show physiological effects of the drug, and it's likely that something similar is going on in the human brain. But it's not clear how strong the effects are, he said — if an animal exposed to pot runs a maze a few seconds more slowly, how does that translate to points on the human intelligence scale?
Jackson and Isen's research, published in the journal Proceedings of the National Academyof Science in February 2016, suggests that whatever marijuana's impacts are, they're dwarfed by the environmental factors that caused the pot use in the first place. Jackson said he suspects the results conflict with the 2012 study in New Zealand because in that study, researchers were following heavier users over the longer term, so the results reflect the problems those users had in childhood rather than problems caused by the pot use itself.
"I think the real question ends up being for kids, 'Should I be more concerned about how marijuana is affecting their brain, or should I be more concerned about what are the things that have led that person to seek refuge in marijuana?'" Jackson said. "What is going on in that 14-year-old's home life?"
Nevertheless, the research in this area is too nascent to draw firm conclusions about whether marijuana use is safe over time, all other things being equal. The National Institutes of Health announced last year that it is launching a longitudinal study of 10,000 children to track the effects of substance abuse, including marijuana exposure, over time. The Adolescent Brain Cognitive Development Study will use neuropsychological testing, as well as brain imaging, to delve into these questions.
The answers are likely to be complicated by ever-changeable factors, such as the strength of marijuana being cultivated, Jackson said. Modern weed has been bred to be higher in THC than strains smoked in previous decades, and those concentrations could matter to the brain.
"I think it's going to be a very long time until we know," Jackson said.
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.