After being pummeled for allegedly marketing to teens, Juul recently made a very public switch to focusing on adult cigarette smokers.

Juul executives have been out there telling people not to vape unless they already smoke. That message was tied to its latest effort to switch gears and pitch Juul as a safer alternative to cigarettes. As it turns out, the Food and Drug Administration didn’t like that either.

The FDA sent its first official warning letter to Juul this week. The 12-page missive is a culmination of the FDA’s work to crack down on youth vaping. It covers Juul’s marketing, smoking cessation claims and use of nicotine salts and high nicotine concentrations.

While details about Juul’s youth marketing tactics have been made public through other investigations, this is the first request to look at the company’s claims that vaping is a safe alternative to smoking. The FDA is requesting that Juul provide documents and information to explain why its vapes are safer than cigarettes. In fact, Juul has been blatantly marketing its vapes as cigarette alternatives.

Its Make the Switch campaign includes claims that its product is “99% safer” than cigarettes, “much safer” than cigarettes, “totally safe” and “a safer alternative than smoking cigarettes,” according to the warning letter. If you visit Juul’s website, the words “smoking alternative” and “cigarette alternative” are everywhere, alongside claims that Juul products could “improve the lives of the world’s 1 billion adult smokers.” Though these messages fall short of saying how vapes improve lives, there’s an implication that smoking is harming their lives and vaping is not harming lives.

That’s a rather bold move, given that the FDA has authority over smoking cessation treatments like patches or gum and that the FDA has not yet reviewed any applications for vapes.

“We are also concerned that parts of the Make the Switch campaign may also convey that switching to Juul is a safer alternative to cigarettes, in that using Juul products poses less risk or is less harmful than cigarettes,” the letter reads.

The recent wave of mysterious lung illnesses also may refute the idea that vaping is less harmful than cigarettes. Six people have died in the U.S. from this illness and there have been at least 450 possible cases in 33 states.

The warning letter also asked for information about a “switching” presentation made to the Cheyenne River Sioux Tribe Health Committee, where Juul representatives claimed that switching to vapes could improve lives, reduce combustible products and eliminate the toxic by-products of cigarettes.

For now, the FDA isn’t demanding that Juul remove these claims, giving Juul the chance to prove its claims. The FDA is asking for studies that prove Juul products pose less risk or are safer than traditional cigarettes, as well as studies that show Juul is effective as a smoking cessation product.