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Every time my mom, a campus supervisor at a Southern California public high school, busts underage kids for pot, the students push back with the same protest: Haven’t you seen the news? Weed is totally harmless! Weed is good for you!

It's true, the benefits of cannabis—from the medical to the sexual—are finally making their way back into mainstream American discourse after decades of vilification. But cannabis is a complicated plant, and its physiological effects on the adolescent brain are still largely unknown—a message teens aren’t receiving.

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"There is growing evidence that repeated exposure to cannabis that begins during adolescence is associated with a decline in IQ of about 10 points when subjects are tested in adulthood,” says Sanjiv Kumra, an associate professor of psychiatry at the University of Minnesota Medical School who has studied cannabis' effects in young people. “This effect is not seen in adult users of cannabis.”

Indeed, study after study has shown that frequent cannabis use among teens can lead not only to a drop in IQ points, but altered cognitive performance and even psychosis. Or as a 2008 study put it, frequent use can spur “lasting neurobiological changes that can affect adult brain functions and behavior."

Since an adolescent's brain is not fully developed, cannabis appears to affect it differently than an adult brain. The thing is, neuroscientists believe our brains are not fully developed until well into our twenties, around age 24 to 26. So at the risk of seeming like a total square, one must ask: Should the minimum legal age for recreational marijuana use actually be much higher?

The Colorado model: "Regulate like alcohol" 

Since Colorado was one of the first states to legalize recreational use of cannabis, many look to the state as a model for future legislation. With the passage of Amendment 64, Colorado set the legal age at 21 and now other states are following suit. By why 21?

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"We generally settled on 21 because that is the age that our society deems appropriate for an intoxicating substance, such as alcohol," said Mason Tvert, director of communication for the Marijuana Policy Project, which helped push through Amendment 64.

Most of the Colorado legalization campaign revolved around the phrase, "regulate like alcohol," which would mean setting the same age limit. But weed and alcohol are two very different substances (one could argue that alcohol is much worse in terms of addiction and related violence, but that's another story), so there's no real reason to treat them the same.

"I support the age limit of 21," said activist Rico Garcia from the Cannabis Alliance for Regulation and Education, who helped with early drafts of Amendment 64. "Let's get you through basic public school before you use copious amounts of cannabis."

Everyone I spoke to agreed that lowering it to 18 or 19 (another age that was on the table simply to keep it off high school campuses), as some activists have suggested, would be problematic for young students, given marijuana's mind-altering effects. "If kids are ADD they don't need to be daydreaming on cannabis," said Garcia. "We don't want it on a campus at all unless it's in a medical context." And yet, raising the age limit was never really on the table.

So the main argument for the current age limit appears to be twofold: Intoxication at age 21 is accepted by society, and there's a pressing need to keep it out of schools—but what about medical concerns?

"We had a number of discussions about how marijuana affects a young person's brain and what is an appropriate age limit," said Brian Vicente, a lawyer at Vicente Sederberg in Colorado, who helped draft the actual legislation. "But it's something our society recognizes as an appropriate time."

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Our society also says that voting, tobacco use, and going to war are appropriate for 18-year-olds, but you need to be 25 to rent a car. Is "society's" opinion really the best measure for a plant that affects us on neurological level?

Is 21 old enough? 

According to the 2014 World Drug Report, cannabis use among 8th to 12th graders rose by 2 to 6 percent from 2008 to 2013, and 36.4 percent of 12th graders reportedly used cannabis in 2013. That same report found that more permissive cannabis regulations are correlated with a decrease in the perceived risk, and in turn, a rise in use. Basically, as the teens tell my mother, "Weed is harmless, duh."

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To further that point, in a federally sponsored survey, 60 percent of high schoolers said they believed marijuana is perfectly safe.

But in an effort to undo the past eighty years of propaganda portraying weed as an evil, hardcore drug (remember, before the days of Reefer Madness, cannabis was used in American medicine), we may have inadvertently forgotten to tell young people that "safe" is an overstatement.

"People absolutely need to learn about the nuances of marijuana," said Tvert. "For example, we tell teens that sex is something adults do, and they need to understand there are risks when you're younger, it should be the same with pot."

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Of course, this isn't just about high school-aged kids. By setting the age at 21, we as a society are saying that 21 is a safe age to use marijuana. But the warnings that apply to school kids may need to be heard on college campuses, too. Here's why.

How weed affects a younger brain

A person's prefrontal cortex—which influences cognitive ability, personality, and decision making—is not fully developed until his or her mid-twenties. And introducing a chemical compound that can alter development can pose risks.

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In order to understand these risks, it's useful to understand how cannabis works in the body.

Essentially, weed is made up of chemical compounds called cannabinoids, the most active and famous being tetrahydrocannabinol (THC). These compounds interact with receptors in the brain's endocannabinoid system, which can influence appetite, memory, brain function, energy, and stress response (which explains why cannabis is associated with all of the above.)

In a teen or young adult brain, however, these interactions can lead to slight alterations in neurological wiring, due to the fact that connections are still developing.

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Kumra, the professor of psychiatry, said he has observed that repeated exposure to cannabis at a younger age is indeed associated with alterations in brain development. "We find evidence of both disruptions in white and grey matter development, particularly in fiber tracts, which project to the prefrontal cortex and brain regions that subserve aspects of cognitive function."

There's also been evidence that the higher the THC levels in a particular dose of weed, the more the brain changes, said Krista Lisdahl, director of the brain imaging and neuropsychology lab at University of Wisconsin, who has researched marijuana's effects on the brain.

She explained, "Basic science shows us higher potency or dose is more likely to alter the endogenous endocannabinoid system in your brain, and this system is important for cognition, stress, and emotional regulation." And because the brain is in its final stages of development in our early twenties, that may be the worst time to introduce marijuana. Especially high potency strains.

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"We have strong evidence that brain structure, especially in the prefrontal and parietal cortices, is still changing until age 25," said Lisdahl. "If we based it completely on science, I would recommend the age limit be 25."

She added that policymakers should think about capping THC potency levels at 15 percent. For comparison, cannabis in the 1960s and 1970s had THC levels around 7 to 10 percent. Today, levels range from 10 to 20 percent in some strains—and concentrates such as dabs can contain as much as 80 percent THC.

So what should be done? 

Despite research that appears to show the negative effects of cannabis on young brains, several experts pointed out that more research is needed in determining the long-term effects.

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"The evidence regarding marijuana use and brain development is disputed," said Dale Gieringer, the director of Cal NORML (National Organization for the Reform of Marijuana Laws). "Experts on adolescent drug abuse are more concerned that youthful potheads will drop out or neglect their studies than that will suffer some kind of brain damage."

Not to mention, as other experts told me, it's unrealistic to believe that upping the age limit would stop young people from using pot. "If you say you should never do it, it's not gonna work," said the Marijuana Policy Project's Tvert. "My dream campaign would be to teach kids 'just wait' because it's not as harmful when you're older."

Tvert said that when Amendment 64 passed, Colorado did launch a public awareness campaign to educate (read: deter) teens and college kids, using the concept of caged lab rats. "They put these steel cages around the state, with the slogan 'Don't be a lab rat,'" said Tvert. The ads also included questions such as, "Who’s going to risk their brains to find out once and for all what marijuana really does?"

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But the campaign backfired, and young people posted photos to social media smoking pot inside or near the cages. The state then quietly ended the campaign.

So how can advocates get the message across? "It's really a matter of creating campaigns that will reach younger people honestly," said Tvert. "We see that more with sex education now, rather than abstinence only campaigns."

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Garcia, from CARE, agreed that more should be done to change how the image of cannabis use is presented to both teens and adults alike. He said that the influx of candy edibles such as gummy worms and lollipops, marketing featuring girls in tight T-shirts with 420 plastered across their chest, and the whole Snoop Dogg association makes the drug seem childish—and that's the wrong approach.

"There are certain things we need to do to keep it out of the hands of children, and secondly, people need to stop acting like children," he said. "A lot of adults and kids need to learn to use cannabis responsibly."

Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.