E-cigarette startup Juul has 60 days to prove it can keep its devices out of the hands of kids, the US Food and Drug Administration said today, or else the company’s flavored products might be taken off the market.

Juul isn’t the only company that the FDA has targeted in a new crackdown on e-cigarettes. It has given similar warnings to five other e-cig companies — MarkTen, Vuse, Blu, and Logic — that, together with Juul, make up almost the entirety of the US e-cig market. The agency has also issued warning letters to over 1,000 retailers that illegally sold Juul to minors.

A Juul spokesperson said that the company plans to “work proactively with FDA” and is “committed to preventing underage use of our product.”

E-cigarettes have been exploding in popularity, but there’s still a lot we don’t know about their health effects. Because they don’t contain as many harmful chemicals, e-cigs were initially heralded as a “healthier alternative” to smoking or a way to help people quit. An exhaustive review by the National Academies of Science, Engineering, and Medicine found that vaping reduces some of the health risks associated with smoking. But there aren’t enough solid studies to show that it’s better than nicotine gum or going cold turkey to kick the habit. And, increasingly, there is concern that e-cigarettes are targeting children who have never smoked to begin with and then become addicted.

Juul, in particular, has been hit with lawsuits — including a nationwide class action lawsuit — claiming that its products are harmful and target teens. Due to JuulSalts, its patented nicotine formulations, Juuls provide a nicotine hit that’s more like smoking a cigarette than other e-cigs. These signature flavored liquids contain higher doses of nicotine than other e-cigs, and they’re all packaged with kid-friendly names like “Fruit Medley” and “Creme Brulee.” (The company later changed these names to just “Fruit” and “Creme.”)

Though the true negative health effects of nicotine are still under investigation, the FDA worries that Juul’s marketing and flavors are largely responsible for its popularity with teens. “I believe certain flavors are one of the principal drivers of the youth appeal of these products,” FDA commissioner Scott Gottlieb said in a speech at agency headquarters.

The FDA started investigating Juul’s marketing tactics in April. Gottlieb has said that he thinks e-cigs are a helpful smoking alternative for adults, but he also says that it’s time for regulators to be tougher on companies to keep kids safe. “Inevitably what we are going to have to contemplate are actions that may narrow the off-ramp for adults who see e-cigarettes as a viable alternative to combustible tobacco in order to close the on ramp for kids,” Gottlieb told The New York Times. “It’s an unfortunate trade-off.”