In 2016, it looked like public health officials had dodged a bullet. Teen use of tobacco products, which had been rising for two straight years following the introduction of e-cigarettes, was on the decline. That relief has been short-lived. Teen e-cigarette use is up 75 percent this year, and the U.S. Food and Drug Administration (FDA), recently declared youth e-cigarette use an “epidemic.”

What changed? One word: Juul. The discrete pod-style vape pen took off in 2017 with a polished design and a prolific social media marketing strategy. Like Google, the brand name has become a verb, with “juuling” now synonymous with vaping. As of August, Juul had gobbled up 72 percent of the e-cigarette market, and the company’s sales are up more than 800 percent from the previous year.

“You couldn’t design a combination of a campaign and a product more perfect to undo all the good that has been done over the last 30 years.”

“Juul is the perfect product for kids because it’s all electronic and modern,” says Stanton Glantz, a professor of medicine at University of California San Francisco and director of the UCSF Center for Tobacco Control Research and Education. “From a public health point of view, it’s a disaster.”

In addition to the product’s appealing design, Juul piggybacked on the trend of using sweet flavors that appeal to kids. The company also changed the formulation of its e-liquid to deliver higher levels of nicotine without the usual harshness, making it even more addictive.

“Juul was the perfect storm,” says Matthew Myers, president of the Campaign for Tobacco-Free Kids. “You couldn’t design a combination of a campaign and a product more perfect to undo all the good that has been done over the last 30 years.”

From its launch in 2015, experts, including the Campaign for Tobacco-Free Kids, say Juul targeted millennials and Generation Z, the vast majority of whom are under 18. Juul promoted parties, hosted vapor lounges at music festivals like Nocturnal Wonderland, and invested heavily in social media advertising through sponsored content on Twitter and Instagram. The posts featured young, attractive models paired with taglines like “The freedom of a #JUULmoment” and “Share a #JUULmoment” (many such ads have since been deleted).

The advertising took off in a way that print ads never could. Teens started posting photos and videos of themselves blowing vape rings, vaping multiple Juuls at once, and tricking friends into hitting a USB stick (Juul resembles a flash drive)—all tagged with #juul, #juulnation, and #doitforjuul. Juul hashtags quickly took over Instagram and Twitter, so much so that accounts now tag unrelated posts with #juul just to get more views.

“Whatever the intent of the Juul people are, whatever they claim, it’s impossible to believe that they didn’t realize that kind of imagery and these kinds of flavors were likely to appeal to kids,” says Myers.

“It’s not just the ads that are getting kids hooked — it’s the flavors.”

Juul, like most e-cigarette companies, says its product is intended to be used as a cigarette substitution therapy by smokers who are trying to quit and claims it never deliberately targeted teens with advertisements. (Juul turned down multiple requests to comment for this article). However, a New York Times interview with an anonymous former senior manager from Juul suggests the company knew its ad campaigns would appeal to teens. He also said that Juul was aware teens were buying the devices as early as 2015. Juul is currently being sued by a mother who says her 15-year-old son became addicted to nicotine because of the product.

But it’s not just the ads that are getting kids hooked—it’s the flavors.

Flavored cigarettes have been banned by the federal government since 2009 because of their appeal to children, but other types of tobacco products were not included in the ruling. Although e-cigarettes like Juul do not technically contain any tobacco, the nicotine they deliver is derived from the plant, so they are classified by the FDA as a tobacco product but not a cigarette. E-cigarette companies have exploited this loophole and developed thousands of flavor variations, many of them sweetened with sucralose to make them taste better. Sweet flavors are not unique to Juul—other companies offer e-liquid flavors like sour candy worms and “unicorn poop”—but Juul plays right along with mango, crème brûlée, and cool cucumber pods (recently changed to creme and cucumber after pressure from the FDA).

This month, the FDA cracked down on e-cigarette retailers and manufacturers for selling and marketing to minors by issuing more than 1,300 warning letters and fines as part of a “nationwide, undercover blitz.” The agency also gave the top five e-cigarette brands, including Juul, 60 days to submit plans describing how they will combat underage use of their products, explicitly mentioning the issue of flavors. If they fail to do so, the FDA warned it would consider revoking the companies’ ability to sell their products. The action comes after the FDA ordered online retailers in May to change the labeling and advertising of e-liquids “resembling kid-friendly food products such as candy and cookies.”

Multiple recent studies have found that teens overwhelmingly start vaping using flavored e-liquids (candy and fruit flavors are the favorites), and flavor is the second-biggest reason they try vaping after curiosity. Perhaps most damning, one recent report suggested that if e-cigarettes didn’t come in flavors, more than 75 percent of teen and young adult vapers would no longer use them.

“They’re making the negative aspects of the drug less aversive and more rewarding to children by making it taste better.”

This is not to say that adults don’t like flavored e-cigarettes (they do), but scientists say young people’s brains are specifically wired to prefer the taste of sweet. “Children, especially during periods of growth, prefer sweets. It’s to attract you to energy while you’re growing,” explains Julie Mennella, a faculty member at the Monell Chemical Senses Center. “I don’t think that the addition of [sucralose] in these candy-like flavors was just by chance. They’re making the negative aspects of the drug less aversive and more rewarding to children by making it taste better.”

Sweet and bitter flavors are shown to counteract each other in the brain, with sweet tastes blocking bitter sensations. In e-cigarettes, this means the sweet flavors mask the bitter taste of nicotine. Sweet tastes also blunt feelings of pain by tapping into the brain’s opioid system—the same network activated by morphine and heroin to relieve pain and provide pleasure. In Mennella’s studies, children who are given sugar water can hold their hands in ice water for longer, and some doctors recommend giving infants a sweet syrup before drawing blood so they’ll experience less pain. It’s possible, health experts argue, that sweet flavors in e-cigarettes could temper the initial harshness and discomfort of inhaling vapor.

“The first time you try a tobacco product, the extent to which you like it or not will be a big determinant as to whether you’ll continue,” says Adam Leventhal, a professor of preventive medicine at the University of Southern California. “An unflavored product might be more aversive and bitter to youth. If you add a flavor on top of that, that might potentially mask or reverse the bitterness or even the sensory harshness of the product, then you might be able to tolerate it better and get past the initial point.”

Sweetness is also reinforcing in and of itself, activating the same dopamine reward system in the brain as addictive drugs like cocaine and nicotine. In fact, there’s evidence that sweet tastes and nicotine magnify one another’s reinforcing qualities and combined are more rewarding than either one alone. A recent study showed that e-cigarette smokers had greater activation in the nucleus accumbens—a key reward center in the brain that has been used to predict drug use and relapse—in response to sweet-flavored e-cigarettes than to sweet flavors without nicotine or to nicotine on its own.

Dana Small, a professor of psychology and psychiatry at Yale School of Medicine who led the research, says that some people actually disliked vaping the nicotine on its own, but when it was paired with the sweet flavor “they went from disliking to liking,” she says. “In those individuals who might have formed a negative preference or aversion to the e-cigarette, this [sweet taste] shifted it into the positive.”

Flavor isn’t the only aspect of e-cigarettes that can increase their addictive potential. The higher the concentration of nicotine and the faster it gets to the brain, the more addictive it is. One 2017 study showed that higher nicotine concentrations in e-cigarettes were associated with teens vaping more—both more times per day and more puffs each time. The research also indicated that higher levels of nicotine in e-cigarettes makes the transition to regular cigarettes more likely.

Juul delivers a bigger dose of nicotine than earlier e-cigarette models, in some cases up to 10 times more. That much nicotine can be harsh to the throat and lungs, but Juul developed a special formula to get around this problem. The company added an acid to the e-liquid to turn the nicotine into a salt instead of a freebase, the form most e-cigarettes use. The new formulation means Juul can deliver a much higher nicotine concentration without the unpleasant feeling in the throat and lungs associated with traditional cigarettes or other e-cigarette products, which can be a deterrent to smoking.

The higher dose of nicotine in Juul products may have a big impact on a teenage brain, which research has shown is more susceptible to the effects of the drug. Until the age of 25, the brain is still growing new connections between neurons while simultaneously pruning out unused ones. Adding a psychoactive substance to the brain can derail this process, says Bonnie Halpern-Felsher, a professor of pediatrics at Stanford University.

“You introduce nicotine to [the brain], or any harmful substance, and it actually changes the chemistry of the brain. And it changes the chemistry of the brain particularly as an adolescent because you’re so malleable,” says Halpern-Felsher. “As you get older, past mid-20s, that process is much harder. You can still become addicted but it’s much harder because your brain has already hardened and developed.”

Juul and other e-cigarette manufacturers maintain that vaping can help people quit smoking and that e-cigarettes are significantly less harmful than traditional tobacco cigarettes. While the jury is still out on these claims, researchers say teens have gotten the message that vaping is less bad for you.

“Adolescents’ perceptions of cigarettes thankfully have soured in a good way,” says Halpern-Felsher. “They believe that cigarettes are bad, not socially normative, harmful, you name it.” However, she says her research suggests that teens do not consider Juul to be like cigarettes, and because the devices have been marketed to help people stop smoking, kids think they’re healthier. “If you’re saying it’s okay to use it to stop smoking then it must be okay for me to use, period,” she says.

“The way that people used to think about e-cigarettes is it’s kind of like a cigarette without quite as much bad stuff. But now it’s quite clear that e-cigarettes are a distinct product with a different risk profile.”

In one of Halpern-Felsher’s studies, 23 percent of high schoolers thought e-cigarettes were not a tobacco product, and 19 percent believed the vapor was from water. More than 40 percent of the teens said that e-cigarettes were safer than regular cigarettes and could help people stop smoking. Notably, students who used e-cigarettes were more likely to believe they are not a tobacco product, were safer than smoking, and were not addictive. Another study revealed that 63 percent of teen and young adult Juul users didn’t know the product always contained nicotine.

Though Juul turned down requests to comment for this story, the company did send information on its initiatives to combat underage use. Juul has invested heavily in revamping its image, including changing its website and social media to solely feature former smokers. It’s also working with Instagram and Facebook to take down posts and accounts that “portray our product in unauthorized and youth-oriented manners.”

As for whether e-cigarettes are really less harmful than regular cigarettes and can help smokers quit, it depends on who you ask. Glantz, the UCSF professor, says that while e-cigarettes have a lower risk of cancer (because of fewer cancer-causing chemicals from tobacco smoke), the likelihood of having a heart attack or lung disease if you are a daily e-cigarette user approaches that of being a daily smoker.

“The way that people used to think about e-cigarettes is it’s kind of like a cigarette without quite as much bad stuff. But now it’s quite clear that e-cigarettes are a distinct product with a different risk profile,” says Glantz. “They’re having effects that add on to the effects of smoking rather than replace it.”

There is evidence that e-cigarettes can help smokers quit, but the numbers are paltry. According to one study, 8.2 percent of smokers were able to quit using e-cigarettes compared with 4.8 percent who quit without them. Any benefits are likely outweighed by the number of new tobacco users that e-cigarettes are initiating. What’s more, two adult smokers have sued Juul claiming the product actually made their addiction to nicotine worse.

Many opponents of e-cigarettes are calling for more oversight, similar to that of traditional tobacco products. “I think the only chance we have is if the FDA firmly comes down and says no flavors [allowed] until you prove they help smokers quit and could be marketed without appealing to kids,” says Myers, from Campaign for Tobacco-Free Kids.

Glantz argues that if Juul and other e-cigarette companies are serious that their products are only to be used to quit smoking, “They should register it as a drug and have it available by prescription so that it’s used under medical supervision as part of a smoking cessation program.”

While Juul is taking strides to comply with the FDA, it may be too little too late. There’s no data yet about how easy or difficult it is to stop using an e-cigarette, which is ironic considering they’ve been touted as a smoking cessation tool. In general, success rates at quitting smoking hover around 7 percent, although the number jumps to nearly 10 percent for smokers under the age of 25. This is bad news for the millions of teens who use e-cigarettes, whose high school rebellion could develop into a lifelong and potentially deadly addiction.