Does using marijuana as a teen make you dumber? Here's what the research says.

Major advocates on both sides of the marijuana legalization debate agree that teens shouldn't use marijuana, but exactly why they shouldn't — and how much of an effect marijuana has on teens in the first place — is a subject of much dispute in the research world.

Over the decades, public figures have associated teen marijuana use with all sorts of bad outcomes. Some groups claim it leads to lower IQ scores. Others have compared pot to more dangerous opiates. News coverage of recent studies seems to support the concern that marijuana is very dangerous for adolescents. But while the available research may at times seem conclusive, experts haven't been able to prove that marijuana directly causes these issues.

But questions surrounding teen pot use are becoming increasingly important as the nation adopts an increasingly favorable look toward marijuana legalization. And with much of the research still in its infancy, this developing field of study could provide ammo for both sides of the legal pot movement as the topic moves to the mainstream.

The research on teen marijuana use is close to unanimous in at least one sense: More frequent marijuana use appears to correlate with various detrimental effects, including worse education outcomes and cognitive deficiencies. These are correlations, though, and researchers have yet to prove a causal relationship. But that doesn't mean there isn't one.

Some experts caution that there's some reason to believe that the relationship between frequent marijuana use and bad outcomes is causal. Researchers know, for instance, that marijuana interacts with receptors in the brain that peak during adolescence and are particularly sensitive to outside exposure. Some of this development continues into the mid-20s, which means even young adults may be at risk to the detriments of pot use.

"There are converging lines of evidence that regular (at least weekly [to] daily) cannabis use during the teenage and young adult years is associated with poorer cognitive function (verbal memory, processing speed, cognitive inhibition, sustained attention)," Krista Lisdahl, director of the brain imaging and neuropsychology lab at the University of Wisconsin at Milwaukee, wrote in an email. "We also see subtle abnormalities in brain structure and function, as well as poorer self-reported mood and sleep quality. The effects are greatest in teens, then young adults, and less so in adults over 25."

Not everyone is as convinced as Lisdahl, and the debate has sometimes played out quite publicly in the research world. For example, one study published in PNAS in 2012 found that people who use marijuana as adolescents tend to have lower IQ scores. But another study, published in the same journal, subsequently concluded the findings could be nearly eliminated by controlling for socioeconomic variables.

Ole Rogeberg, who authored the study that controlled for socioeconomic variables, elaborated on his findings in an email: "The data tells us that kids from difficult backgrounds and kids with social and behavioral problems tend to start smoking marijuana earlier and more heavily. The data also tells us that early and heavy marijuana users on average go on to do worse in various ways than teens who abstain. What we are less sure of is why."

A similar debate took off when a big study published in The Lancet found daily marijuana users ended up having a range of bad outcomes, including a 62 percent reduced chance of finishing college. The study was held up as credible for two reasons: It employed a lot of rigorous controls that ranged from the economic to the social, and the bad outcomes worsened as teens reported more marijuana use. Combined, the controls and trajectory of the findings suggest a strong relationship between pot and some bad outcomes.

Joseph Palamar, a professor at New York University's Langone Medical Center who has conducted his own research into teen pot use and the stigma surrounding it, isn't so sure. While he praised the Lancet study for its strong controls, he said there could still be unaccounted variables, particularly any stigma surrounding pot use, influencing the results.

It's possible, for instance, that teachers are biased against students who use marijuana and, as a result, complicate pot users' education. "Since marijuana use is an illegal behavior, it may be that teachers pinpoint users and start getting on their cases," Palamar explained.

Edmund Silins, one of the authors of the Lancet study, called Palamar's suggestion "plausible" and acknowledged the difficulty in this field of study. "You have highlighted a good example of how complex this area of study is," he wrote in an email. "It is not possible to control for every factor that might explain the association between cannabis use and harms. I wish we could."

But Silins added, "My view is that including an additional control factor [such as stigma] over and above 53 other factors that had already been included in the analysis is probably unlikely to make much difference. But that hypothesis remains to be tested through research."

At the same time, there's general agreement that teens shouldn't use marijuana. Part of that goes beyond the research on pot's effects on the brain: Regardless of how bad the drug is for someone's mental health, teens are at risk of losing control of their marijuana use at a particularly important point in their lives.

A less tangible impact of marijuana isn't its effect on physical or mental health, but rather its effect on productivity. If a teenager gets caught up in using pot every day, she might spend so much of her time impaired that she'll never, for example, be able to concentrate on her schoolwork, falling into a potentially bad habit and career path for the rest of her life.

"The main risk of cannabis is losing control of your cannabis intake," Mark Kleiman, a drug policy expert at New York University's Marron Institute, said. "That's going to have consequences in terms of the amount of time you spend not fully functional. When that's hours per day times years, that's bad."

Jon Caulkins, a drug policy expert at Carnegie Mellon University, put it another way: "At some level, we know that spending more than half of your waking hours intoxicated for years and years on end is not increasing the likelihood that you'll win a Pulitzer Prize or discover the cure for cancer."

This could explain some of the poor educational outcomes associated with teen marijuana use. It's not that pot permanently makes teens dumber, but these kids may be high so often that they simply can't apply themselves in school. As more research comes out, we should get a clearer answer to this question: Is pot actually damaging a teen's brain, or does the high only make someone temporarily impaired?

As the debate continues, more research is sure to come on the full effects of marijuana. As with other developing fields of research, one can expect that many, many studies will come out in the next few years — and some will contradict each other, while others will concur. Over time, a clearer consensus should appear on the drug's effects.

"[W]e do need additional research following young teens over time to ensure some of these findings are not there before the drug use," the University of Wisconsin's Lisdahl wrote in an email. "The scientific community, led by the National Institutes of Health, is in the process of designing a large, multi-site, longitudinal study that will measure the impact of cannabis, nicotine, alcohol use, and other drugs on the developing brain before drug use starts and into early adulthood."

This type of longitudinal research requires years and even decades of follow-up to fully flesh out. And researchers can't rely on old data to conduct longitudinal analyses: Since marijuana has gotten more potent over the decades — and potentially more harmful to the teen brain — researchers will need to launch new analyses with the modern, more powerful drug to gauge its full effects.

"[T]he key problem with all of these studies is that they are looking at the impacts of low-potency [marijuana], not the 15- to 20-percent THC potency stuff people are smoking today," Rosalie Pacula, a co-director at the RAND Drug Policy Research Center, wrote in an email. "The mental health effects of 5-percent THC (and some CBD) are expected to be quite different than the mental health effects of high-THC, low-CBD cannabis."

But with the nation already in majority support of legalization, voters may end up moving more quickly than the science on this issue. Still, for that to matter, full legalization would need to increase teen pot use, therefore exposing more teens to a potentially dangerous drug — and whether legalization increases teen use is unclear.

Marijuana legalization might not increase teen pot use

Marijuana's potential risks to teenagers don't necessarily justify keeping pot illegal. Even in states with legal marijuana, buying and possessing the drug remains illegal for anyone under 21. And there's no conclusive evidence so far that legalizing marijuana increases teen pot use.

Silins, of the Lancet study, pointed to previous research that found teen pot use didn't increase in states that legalized medical marijuana — which can often look a lot like legalization, since regulations are so weak that practically anyone can get a medical card and buy pot.

But the research didn't account for variation in medical marijuana laws — whether the state allows dispensaries that sell pot, home growing, or neither. A comprehensive study of medical marijuana laws from researchers at the RAND Corporation found policies that allow medical marijuana dispensaries correlate with increases in overall marijuana use and dependence for adults 21 and older but only rises in dependence among youth. More research will flesh out the full effects.

In Colorado, teen pot use declined after the state legalized medical marijuana in 2001, dispensaries began opening after 2007, and possession was fully legalized for adults 21 and older in December 2012. But the available data is before retail sales began in 2014, and it's possible the full legalization of sales and the commercialization that comes with it could have a different effect on adolescent pot use.

Opponents of legalization, such as Kevin Sabet of Smart Approaches to Marijuana, argue that the commercialization of legal pot encourages for-profit retailers to market their products to young people — to get them hooked on the drug early. He points to marijuana-infused edibles, which often come in candy form, as products that appeal to youth. "Kids are getting the message that [marijuana use] is okay," Sabet said.

Marijuana legalization advocates, on the other hand, argue that a legalized, regulated model can do more to decrease teen pot use than prohibition. They point to the past few decades of the war on drugs, which didn't significantly reduce and hold down illicit drug use, and contrast it with the decades-long decline of teen alcohol and tobacco use. Public health officials and experts widely attribute the decline of alcohol and tobacco use to more stringent regulations, including the harsher enforcement of age restrictions at vendors, and education campaigns that teach teens the risks of drug use.

"No one wants kids using marijuana. We should be doing everything we can to prevent it," Mason Tvert, spokesperson for the pro-legalization Marijuana Policy Project, said. "I think [prohibition] is utterly failing to prevent it."

There's some reason to believe that carrying out similar regulations and education campaigns against legal marijuana will be more difficult. While it's very easy to tell teens about the clear risks of alcohol and tobacco, it's difficult to establish a campaign line for pot that won't seem hyperbolic or false, since the science on marijuana's health effects isn't settled and the effects of the drug, if they're real, can take years or decades to culminate. It's much easier, then, to point out the dangers of tobacco or alcohol without coming off as sensationalist.

Some states are trying to make anti-marijuana campaigns work. As the first state to fully legalize marijuana for possession and sales, Colorado attached the beginning of legalization to an education campaign that will attempt to warn teens of the potential risks of marijuana while acknowledging the science isn't settled. This balance of embracing legalization while trying to avoid teen pot use, researchers say, could be the right way to adopt reforms.

"[G]iven the findings of the Lancet paper, it would be wise that any cannabis legislative reforms be carefully assessed to make sure they decrease adolescent cannabis use and prevent potentially adverse effects," Silins wrote in an email. "There are likely to be some advantages to decriminalizing or legalizing cannabis, for example, in terms of reduced criminality, but these need to be weighed up alongside evidence of its potential harms."