Dumb and dumber? Teen marijuana use linked to lower IQ in later life
Jokes aside about tuned-out stoners who can’t find their car, some experts are asking, what if marijuana actually makes kids dumb?
Earlier this month, three researchers at the National Institute of Drug Abuse published an article in the New England Journal of Medicine surveying the current state of the evidence. According to their report, marijuana use in adolescence and early adulthood may measurably lower users’ IQ decades later down the road.
They conclude there is reason to believe marijuana may permanently harm the adolescent brain. Until the age of 21, the piece notes, the brain “is intrinsically more vulnerable than a mature brain to the adverse long-term effects of environmental insults.”
Given the rapid pace of marijuana legalization, researchers are noting an increased urgency to do research on the developing brains of teen users. Washington and Colorado have both legalized recreational marijuana use, and legalization is actively being considered in 14 additional states. While none of these states propose making pot legal for minors, destigmatization and greater ease of access have already resulted in heavier use among youths in Colorado. As marijuana is increasingly normalized and seen as relatively harmless, some experts doubt whether we know enough to justify rapid shifts in policy and behavior in pot usage.
“There is a lot we know and a lot we don’t,” said Wilson Compton, deputy director of the National Institute of Drug Abuse and a co-author of the NEJM paper, in an interview. “We think it’s very important to understand risk and protective factors and to understand how significant any one of them is.”
Public opinion, for the moment, may be racing faster than science can keep up.
Last fall, Gallup reported that a solid 58 percent of Americans favored legalizing pot. And an NBC-Wall Street Journal poll this spring found that 49 percent saw tobacco as harmful, while 24 percent said the same of alcohol, 15 percent of sugar, and just 8 percent of marijuana.
Shifting perception is quickly translating to youth drug usage in Colorado, which became ground zero for pot legalization in 2012 when the state voters chose to legalize and tax the drug.
The Office of National Drug Control Policy reported last year that one in four Boulder County high school students now use pot — more than three times the national average.
And the numbers are shifting fast. In Adams County, a Denver suburb, high school marijuana use jumped from 21 percent in 2008 to 29 percent in 2012. Middle school pot use in Adams County jumped 50 percent during that period — from 5.7 to 8.5 percent.
No one is certain what all of these behavior changes mean for the long-term health of kids who become heavy pot users in adolescence, but some researchers think they have a good hunch.
One of the most critical insights comes from a 40-year, ongoing study in Dunedin, New Zealand, where researchers have for years followed the lives of 1,037 children born there during a one-year span in 1972 and 1973.
The kids, now adults, were tested at two-year intervals throughout their childhood and subsequently at ages 21, 26, 32 and now 38. Retention at each stage of the study has been remarkable: At the last interval, they got to 95 percent of the original group. Every angle of physical, mental and social health is examined, and researchers also interview friends and relatives. The result is an unprecedented data trove.
Arizona State University psychology professor Madeline Meier and several co-authors at Duke University used Dunedin’s data to check the effect of adolescent marijuana use, publishing their findings in 2012 in the Proceedings of the National Academy of Sciences.
Those who regularly used marijuana as teens, the study found, lost significant IQ points between their 13th and 38th years. Friends and associates also reported more cognitive problems among regular pot users, and even those who quit did not entirely regain all the ground they had lost in their youth.
The data is so rich that Meier and her colleagues were able to deflect two challenges to their conclusions, and so far no more have surfaced.
One critic from Norway published a critique suggesting that socioeconomic differences may have been the real culprit. Perhaps, he posited, people with duller careers and less stimulating associates had their IQs artificially boosted during their schooling years, and then failed to maintain mental growth as they aged.
Meier and her colleagues went back to the original data and broke it down again, this time focusing strictly on middle class kids, and found that the results held firm.
Another challenger suggested that kids who used pot were naturally lacking in self-control, and thus likely to see their IQ slip as they aged.
Again re-analyzing the Dunedin data, the researchers demonstrated that marijuana’s impact on IQ took place regardless of how much self-control the subject had in childhood.
Meier readily admits the limits of this kind of observational study, which — unlike controlled, clinical studies — cannot determine causation and is often confounded by unseen variables.
“With observational data there could always be an alternative explanation,” Meier said. “However, we did rule out some of the best and most plausible explanations.”
One of the key findings was that the IQ reduction does not occur if the user began smoking marijuana after adulthood. This, Meier says, has led some to see validation in the study for legalization among adults.
“The association seems clear but causal mechanisms not fully understood,” Wilson Compton said. “What we need is additional work.”
The NEJM article by Volkow, Compton and their co-authors cites research showing “impaired neural connectivity” among users who began smoking pot in their teens — including areas of the brain that affect alertness, learning and memory. They also cite studies showing reduced function in the prefrontal networks, which manage conscience and self-control.
All of this is not really surprising, they argue, since the developing brain is peculiarly vulnerable to damage in adolescence and early adulthood.
The NIDA team is currently planning (and arranging funding for) a study that will follow 10,000 children from age 10 through adulthood, looking at the impact of numerous substances and behaviors on the brain. The team will do biological tests and interviews, as well as functional magnetic resonance imaging to see what the brain is doing in real time.
The key to such an ambitious study, Compton said, will be follow-up rates. Many studies struggle to keep track of people over many years, he said, but there are models for what works. Persistence is critical, he said, because tracking down people who move is tough.
“The science and the art is to not make it too burdensome,” he says. “You have to make it interesting and important enough for them that they will be willing to continue.”
By the time the new study is funded, launched and completed 20 years later, an entire generation will have grown up under shifting attitudes toward and usage of marijuana.
And, if Meier and her colleagues are right, many of these newly minted adults will be carrying permanent mental handicaps acquired in the experiment.
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