A spokesman for Juul said the company is reviewing the warning.
The FDA's warning letter refers to several statements, including ones discussed in testimony from a July 2019 Congressional hearing.
According to that testimony, a Juul representative speaking with students in a school presentation stated that Juul "was much safer than cigarettes," that "FDA would approve it any day," and that the device was "totally safe."
The Juul rep also urged a student to mention Juul to a nicotine-addicted friend "because that's a safer alternative than smoking cigarettes, and it would be better for the kid to use," according to the FDA.
The company rep claimed that the FDA would announce "very soon" that "[Juul] was 99% safer than cigarettes," according to the testimony.
The FDA warning also cited a letter from Juul's CEO that appeared on the company's website and in an email sent to a parent who complained that the company had sold products to her child. The CEO's letter stated: "[Juul's] simple and convenient system incorporates temperature regulation to heat nicotine liquid and deliver smokers the satisfaction that they want without the combustion and the harm associated with it."
The FDA also noted a presentation to the Cheyenne River Sioux Tribe in South Dakota. In that presentation earlier this year, Juul said its device is "a smart, really well thought-out alternative to smoking" that will improve "the lives of the world's one billion adult smokers." The company added in the presentation that the "elimination of combustible cigarettes is crucial to reduce risk of harm."
Acting FDA Commissioner Dr. Ned Sharpless sharply criticized Juul's marketing practices in an agency news release.
"Regardless of where products like e-cigarettes fall on the continuum of tobacco product risk, the law is clear that, before marketing tobacco products for reduced risk, companies must demonstrate with scientific evidence that their specific product does in fact pose less risk or is less harmful," he said.
"Juul has ignored the law, and very concerningly, has made some of these statements in school to our nation's youth," Sharpless said in the release.
He said the FDA has put the e-cigarette industry on notice that the agency will take "even more aggressive action" if the "disturbing rise" in vaping among young people continues, especially through the use of flavors that appeal to kids.
"We will continue to scrutinize tobacco product marketing and take action as appropriate to ensure that the public is not misled into believing a certain product has been proven less risky or less harmful," Sharpless said. "We remain committed to using all available tools to ensure that e-cigarettes and other tobacco products aren't being marketed or sold to kids."
The FDA gave Juul 15 working days to outline its plans to comply with FDA regulations or risk further action.
"We are reviewing the letters and will fully cooperate," said Juul spokesman Ted Kwong.
The FDA also asked Juul to explain why it uses nicotine salts - which were said during the Congressional hearing to mask the harshness of nicotine. The company was also asked to explain why its products have a nicotine concentration of 5%, which the FDA said could increase their addictiveness.