And while many smokers have swapped tobacco for a plastic gadget that emits a vapour inhaled like smoke, the "e-cig" faces a threat in Brussels and Washington, where policymakers fret it exploits a legal grey zone.
The e-cig claims to mimic the look, feel and flavour of the real thing -- minus its tar, ash, smoke and most of its toxins.
The gadget contains a liquid that is heated and inhaled as a vapour. The liquid usually has propylene glycol, nicotine and flavourings; there are also non-nicotine versions.
E-cigarettes first emerged in China in 2003 as an alternative to tobacco, which is said to kill nearly six million people a year and inflicts tens of billions of dollars annually in health and productivity costs.
Smokers, anti-tobacco campaigners, policy advisers and many medical practitioners have hailed the device as a valuable aid to quitting tobacco, whose perils have been known since the mid-1950s.
Others, though, question the e-cigarette's safety.
"Vaping", as the practice is called, is banned in nearly a dozen countries, led by Latin America, and the World Health Organisation (WHO) has "strongly advised" against it.
"The potential risks they pose for the health of users remain undetermined," the UN's health organ states in guidelines on its website, adding the device's safety "has NOT been scientifically demonstrated".
European lawmakers want to classify e-cigarettes as medicinal products that can be sold only in pharmacies like nicotine patches, chewing gum and other tobacco-quitting aids.
For now, e-cigs can be sold on the internet or in specialist stores, depending on the country -- in some places also in tobacco shops and pharmacies.
Opponents say the move to regulate the e-cig as a medicinal product will limit availability, push up prices and force millions of nicotine addicts back to traditional cigarettes.
In the United States, the Food and Drug Administration (FDA), which regulates tobacco products, is also expected to unveil proposals in October to govern the sale of e-cigarettes.
"There is no doubt in my mind that e-cigarettes are saving the lives of smokers who have switched to them, and persons around them who would have been exposed to... tobacco smoke," Joel Nitzkin, an American physician and tobacco policy adviser, told AFP.
"E-cigarettes should be considered 'recreational' alternatives to cigarettes for people who have the urge to smoke but would rather not expose themselves and those around them to the many harmful chemicals in cigarette smoke."
Researcher Gerry Stimson of London's Imperial College and Clive Bates, a former director of UK-based Action on Smoking and Health, in a study last month described the e-cig as "a very low risk alternative to cigarettes, used by smokers as a pleasurable way of taking the relatively harmless recreational drug nicotine".
'Vaping': A legal haze
Regulations on e-cigarettes differ, though, between countries. In most, it is still legal to vape in public places; in others, vaping is restricted and the gadget itself, and advertisements for it, may be banned.
Affordability also varies. In France, for example, vaping works out about a third cheaper over a year than traditional smoking. In countries where tobacco is cheaper, the cost benefits would be less.
A recent New Zealand study said tobacco-free electronic cigarettes have proved as effective as nicotine patches at weaning smokers off their habit, but both techniques were only modestly successful.
Other researchers have expressed concern that non-smokers may get hooked on nicotine through e-cigarette use, or that the gadget would keep people addicted to nicotine who might otherwise have quit.
Nicotine can be harmful to children, pregnant women and adults with heart disease.
The US Centers for Disease Control and Prevention (CDC) in September noted a "deeply troubling" doubling of e-cigarette use among American teenagers, and top justice officials from 40 American states have urged the FDA to be tough in its regulation of the industry.
Out of 1.78 million Americans aged 11-18 who are said to have vaped in 2012, some 160,000 had never even smoked a conventional cigarette, said the CDC. The gadget may get children hooked on nicotine, "a highly addictive drug," it said.
Specialists say that with e-cigarettes, there is no tar buildup on the lungs as with traditional cigarettes, nor are there trace chemicals that come from burning paper or growing tobacco.
But exactly what goes into the product and the conditions in which it is made can be hazy.
And whether its bouquet of chemical gasses has an effect on the lungs or other organs remains unknown: there have been only a few health studies, none of them long-term.
E-cig makers and users have submitted petitions with tens of thousands of signatures against the planned EU regulations, which they say will condemn many of the continent's seven million "vapers" to a premature death from cigarette smoking.
Several associations have called for pickets in Strasbourg, France on October 7, the day before the European Parliament is due to vote.