US says vaping market-leader JUUL's advertising was unlawful

US health authorities on Monday accused market-leading e-cigarette maker JUUL of ignoring the law and warned it to stop advertising itself as a less harmful alternative to smoking, noting in particular the company's youth outreach efforts.

The Food and Drug Administration's move is the latest development in its ongoing investigation of JUUL's practices, and comes amid a mysterious US outbreak of severe lung disease linked to vaping that has claimed at least five lives.

Federal officials have yet to identify a single substance behind the wave of illnesses, which has affected hundreds of e-cigarette users and left several teens in induced comas.

"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," said Ned Sharpless, the FDA's acting head.

"JUUL has ignored the law, and very concerningly, has made some of these statements in school to our nation's youth," he added.

The warning letter identified several problematic statements made by a JUUL representative speaking at a school at an unspecified date, including that the product was "much safer than cigarettes" and that "FDA would approve it any day."

The representative also told a student they "should mention JUUL to his friend ... because that's a safer alternative than smoking cigarettes, and it would be better for the kid to use," the FDA said.

The agency also expressed concern about the company's "Make the Switch" campaign aimed at the Cheyenne River Sioux Tribe, and has sought a written reply from JUUL.

The relative harm of vaping versus smoking is contested.

E-cigarette users aren't exposed to the estimated 7,000 chemical constituents present in combustible cigarettes, many of them cancer causing.

But the liquid vapor they inhale contains highly addictive nicotine, and a variety of other substances classed as "potentially harmful" in a landmark 2018 study compiled for Congress.

It might also contain traces of metal from the coil used to heat the liquid or other parts of the device, and experts had warned well before the current wave of illnesses in the US that understanding the long-term effects might require decades more of data.

As far as the severe lung disease probe is concerned, New York state has said that counterfeit cartridges containing cannabis products are the focus of its investigation.

These products contained vitamin E oil which is harmful when inhaled.

But federal authorities stress that no single substance has yet been found to be behind all the cases, and some of those who have fallen ill have reported only using nicotine products, not THC.