The synthetic substances - which have a chemical structure similar to that of nicotine - are not subject to U.S. tobacco and vaping regulations that are designed to control traditional nicotine, a highly addictive drug.

That means manufacturers can sell vapes containing synthetic nicotine analogues such as 6-methyl nicotine in the United States without seeking authorisation from the FDA - a process that can be costly, time-consuming and is often unsuccessful.

Big tobacco firms like Altria Group and British American Tobacco have already lost substantial U.S. sales to an influx of disposable vapes containing traditional nicotine that are being illegally sold without FDA authorisation.

Altria, the maker of Marlboro cigarettes in the United States, highlighted the emerging use of 6-methyl nicotine in vapes and other smoking alternatives in a May 9 letter to the FDA, according to a copy of the correspondence posted on its website.