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The Food and Drug Administration has launched what it calls a “blitz” to stop retailers from selling e-cigarettes to underage kids, with a special focus on the "wildly popular" Juul, a vaping device that looks like a flash drive.

The FDA said it warned 40 stores about selling the product to children under 18 and said it had persuaded eBay to control listings.

The agency asked the makers of Juul to turn over marketing research and other documents to help explain why the product has so quickly taken off with teens.

“As part of the FDA’s responsibility to protect kids and significantly reduce tobacco-related disease and death, these are the first steps in a new effort aimed at stopping youth use of e-cigarettes,” the FDA said in a statement.

Some of the new electronic nicotine delivery systems “have become wildly popular with kids,” FDA commissioner Dr. Scott Gottlieb said in a statement.

That's no exaggeration. Juul controls more than 54 percent of the e-cigarette market, according to the latest retail share data from Wells Fargo Securitues and Nielsen.

"These products are also more difficult for parents and teachers to recognize or detect. Several of these products fall under the Juul brand, but other brands, such as myblu and KandyPens, that have similar characteristics are emerging," Gottlieb said.

Juul and other products that look like flash drives don’t produce the characteristically visible vapor that other e-cigarette products do. That makes them easy to conceal and more attractive to kids, Gottlieb said.

Plus, they typically deliver more addictive nicotine than other vaping devices.

Last week, public health groups including the American Heart Association, American Lung Association, American Academy of Pediatrics, the Campaign for Tobacco-Free Kids, Truth Initiative and the American Cancer Society Cancer Action Network, wrote the FDA to ask the agency to remove Juul from the market until it undergoes FDA review.

“Educators report widespread use of Juul in school bathrooms, hallways and even classrooms,” the Campaign for Tobacco-Free Kids said in a statement.

The FDA is not going that far but said it would try to stop sales to minors.

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“First, we’re announcing that the FDA has been conducting a large-scale, undercover nationwide blitz to crack down on the sale of e-cigarettes — specifically Juul products — to minors at both brick-and-mortar and online retailers,” Gottlieb said.

“The blitz, which started April 6 and will continue to the end of the month, has already revealed numerous violations of the law.”

The FDA said it issued 40 warning letters to various businesses, including 7-11 stores in Maryland, Virginia and Massachusetts, as well as vaping retailers and gas station and other convenience stores across the country.

“Today’s action should serve to put retailers on notice to stop selling products to minors,” Gottlieb said.

Advocates welcomed the action but said it did not go far enough.

“Today’s announcement is a step in the right direction but the FDA needs to accelerate its actions when it comes to regulating e-cigarettes like Juul and remove flavors known to entice youth,” said Dave Dobbins of the Truth Initiative, a group that arose out of a multistate settlement with tobacco makers.

“Keeping e-cigarettes on the market without first evaluating them is putting an entire generation of young people at risk of addiction.”

Groups said flavors such as mango and cool cucumber were enticing children to try e-cigarette products.

“The FDA can and should take immediate action to address these issues and, moving forward, the FDA should utilize this authority to prevent the introduction of kid-friendly tobacco products in the first place, rather than taking action after they become popular with kids,” said Matthew Myers, president of the Campaign for Tobacco-Free Kids.

The FDA said in 2014 it would regulate e-cigarettes and banned the sale of e-cigarettes to anyone under 18 in 2016.

But last summer, Gottlieb extended the timeline for applications for new e-cigarette clearance by the FDA to 2022, delighting e-cigarette makers and infuriating medical groups such as the American Lung Association.

Gottlieb said he recognizes that e-cigarettes can help smokers wean themselves off burnt tobacco products. “But we’ve got to step in to protect our kids,” he said.

“We also recently contacted eBay to raise concerns over several listings for Juul products on its website,” Gottlieb added.

“We’re thankful for eBay’s swift action to remove the listings and voluntarily implement new measures to prevent new listings from being posted to the web retailer’s site. Our overarching goal — one we hope everyone shares — is to make sure Juul, and any other e-cigarettes or tobacco products, aren’t getting into kids’ hands in the first place.”

I hope these actions send a clear message to all tobacco product manufacturers/ retailers that the #FDA is taking on this issue w/ urgency, and if kids are flocking to your product or you’re illegally selling these products to kids, you’re on FDA’s radar: https://t.co/vlwBMXvLK1 — Scott Gottlieb, M.D. (@SGottliebFDA) April 24, 2018

And the FDA asked the makers of Juul for marketing documents and any studies the company may have conducted.

“The information we’re requesting includes: documents related to product marketing; research on the health, toxicological, behavioral or physiologic effects of the products, including youth initiation and use; whether certain product design features, ingredients or specifications appeal to different age groups; and youth-related adverse events and consumer complaints associated with the products,” Gottlieb said.

“We don’t yet fully understand why these products are so popular among youth. But it’s imperative that we figure it out, and fast.”

Juulsaid it would cooperate to some degree. "Juul Labs agrees with the FDA that illegal sales of our product to minors are unacceptable,” the company said in a statement.

“We already have in place programs to prevent and, if necessary, identify and act upon these violations at retail and online marketplaces, and we will announce additional measures in the coming days."

E-cigarettes use a device that delivers fluid laced with nicotine and flavors, creating a smoke-like vapor.

They’re promoted as a safer way to use tobacco — e-cigarette use is called vaping — and as a way to quit smoking. But public health groups, the surgeon general's office, and the Centers for Disease Control and Prevention worry about evidence that vaping can get teens addicted to nicotine, and will lead them to smoking cigarettes.

Many studies support the theory that kids who vape are more likely to go on to use other tobacco products and studies show that vaping delivers potentially harmful chemicals.

The CDC says 20 percent of high school students have used at least one tobacco product recently, most of them e-cigarettes.

Tobacco use is the No. 1 cause of both heart disease and cancer.