Many users of JUUL—the nation’s most popular e-cigarette—may not know that its e-liquid contains an addictive chemical. According to a paper published today in the journal Tobacco Control, 63 percent of users surveyed were unaware that the trendy vaporizer product always contains nicotine. (Some flavored e-cig cartridges for other devices don’t have nicotine in them.)

Like a svelte version of the 2001: A Space Odyssey monolith, the JUUL is a sleek nicotine delivery device that’s extremely popular with young people. It dominates the market, recently securing 54 percent of electronic cigarette sales as of this month, and has sold more than a million units. The verb “juuling” has even entered the popular lexicon.

But the JUUL is not too cool for school. In fact, its small size, minimal odor, and resemblance to a flash drive make it much easier for teens to conceal its use than regular cigarettes. Students even brag about juuling in class. The problem is many teens don’t seem to know the product contains addictive compounds, according to this new research from the Schroeder Institute at Truth Initiative, an anti-tobacco nonprofit. (JUUL says on its website that you have to be 21 to purchase the device.)

For the paper, the group surveyed a nationally representative sample of 1,012 people between 15 and 24 years old. Participants were shown photos of a JUUL device without logos and asked if they recognized it—25 percent said yes, and eight percent (about 80 people) said they’d used the device within the last 30 days.

However, only 37 percent of those 80 users said they knew JUUL’s disposable cartridges always contain nicotine, and the amount is roughly equal to what’s in a pack of cigarettes. (According to their site, “Each JUULpod contains 0.7mL with 5 percent nicotine by weight at time of manufacture which is approximately equivalent to 1 pack of cigarettes or 200 puffs.”)

This is problematic because, according to a 2009 review in Pharmacology & Therapeutics, nicotine use has been shown to impact development of the limbic system in adolescent brains; this area regulates emotions and long-term memory. According to a 2015 report in the American Journal of Preventive Medicine, nicotine exposure has also been linked to “altered development of cerebral cortex and hippocampus,” two brain regions connected to memory, cognition, language, as well as other roles in consciousness.

So it’s important that young people know what they’re putting in their bodies—and it appears many aren’t even aware that there’s nicotine in these products, even though it’s listed right on the packaging and on the website.

Mark Rubinstein, a professor who studies adolescent medicine at University of California San Francisco, was surprised by this finding. “The advertising is pretty clear about the nicotine content, most of my patients seem to be aware of it, and know the 5 percent [nicotine] value from the ads,” Rubinstein says.

A spokesperson for the Truth Initiative said some teens may not even see a JUUL package. “We know that JUUL always contains nicotine, and while the JUUL pod packaging says it contains nicotine, we have heard numerous anecdotes detailing that many youth and young adults are using JUUL without ever seeing a package. And even for those that do handle an actual package—they never read the fine print.”

Watch More From Broadly:

“But I don’t really see anything troubling about the brand identification beyond the fact that this particular product is so appealing to kids,” Rubinstein says. “It is interesting because the flavors are not as numerous or kid-friendly as some other brands.” And yet, teens are using them in classrooms.

The JUULpods come in eight flavors, including Fruit Medley, Cool Cucumber, Mango, and Crème Brulee. The device uses nicotine salts, rather than free-base nicotine, which the company says “delivers satisfaction akin to a traditional cigarette.”

But that salt formulation is what’s most concerning to some people, like Michael Siegel, a professor of Community Health Sciences at Boston University.

“The nicotine formulation of JUUL is unique because it is delivered rapidly into the bloodstream in a pattern that simulates smoking,” Siegel explains in an email. “Therefore, JUUL has addictive potential, unlike most other types of vaping devices.”

Siegel says he thinks the results are valid, in spite of the survey size. “The greater implication is that we need to educate youth about vaping in general and the JUUL in particular,” he says.

When reached for comment, a company spokesperson said: “JUUL Labs is fully committed to raising awareness of the dangers of nicotine use among adolescents. We are investing significantly to combat teenage use, and we welcome the opportunity to collaborate and engage with community leaders, parents, and teachers on education efforts.”

Due to Food and Drug Administration regulations, JUUL cannot and does not claim its devices are safer than cigarettes. But the Truth Initiative is demanding more from the FDA. Last month, they filed a federal lawsuit against the FDA along with the American Academy of Pediatrics, American Cancer Society Cancer Action Network, and American Lung Association.

The groups are pushing the agency to expedite regulatory guidelines for e-cigs that the FDA postponed until 2022. “[T]he FDA’s August 2017 decision to exempt e-cigarettes and cigars from agency review for years to come is unlawful and harms public health,” Truth Initiative said in a statement.

“Preventing underage use of our product is a top priority,” JUUL Labs said. “We hope to partner with those who want to prevent underage use of JUUL and encourage people to reach out to us.”

Follow Troy Farah on Twitter.