An average of 10 new electronic cigarette brands and more than 240 new flavours have entered the internet marketplace every month over the past two years, according to a study by British Medical Journal.
Yet, recent research has produced conflicting evidence as to whether e-cigarettes, promoted as a safer and healthier alternative to conventional cigarettes, are indeed so.
At present, the device has 466 different brands, each with its own website, and 7,764 unique flavours, including gummy bear and marshmallow, reveals the study published in the journal Tobacco Control and funded by the US National Cancer Institute.
Researchers at the University of California, San Diego School of Medicine, who conducted this first comprehensive survey of e-cigarettes online sales from 2012 to 2014, believe their findings underscore the complexity in regulating the rapidly growing market for the electronic nicotine delivery devices.
E-cigarettes deliver a nicotine-containing aerosol, popularly called "vapour", to users by heating a solution commonly consisting of glycerin, nicotine and flavouring agents.
E-liquids have conventional flavours such as tobacco or menthol, as well as flavours inspired by fruits, desserts, candy, alcoholic drinks, and snacks or meals.
The product, reportedly developed in Beijing in 2003, has stoked controversy about whether it improves or worsens public health.
"Some consider [e-cigarettes] promising products to help smokers quit traditional cigarettes, while others believe they will re-normalise smoking, which will keep more people smoking," says the study's lead author Professor Zhu Shu-hong, director of the Centre for Research and Interventions in Tobacco Control at UC San Diego in the US.
In Hong Kong, e-cigarettes containing nicotine and marketed as nicotine replacement therapy are pharmaceutical products under the Pharmacy and Poisons Ordinance and must be registered before sale.
The possession or sale of unregistered pharmaceutical product, and the possession without authority of the inhaler which is considered a Part I poison, are both liable on conviction to a HK$100,000 fine and two years' imprisonment.
But with the online market for e-cigarettes on fire, regulating the use of the product in this city could be tricky.
The UC San Diego researchers, who trawled English language websites marketing e-cigarettes, documented a shift in the marketing of the devices, with newer brands selling customisable e-cigarettes that look more like pens, flashlights - even a violin - rather than an old-fashioned tobacco cigarette.
Older brands were more likely to claim that e-cigarettes were healthier or cheaper than smoking, or that they could help people quit smoking. Newer brands focused on consumer choice, such as flavours or models.
"It almost seems that newer brands don't want to be compared to cigarettes, which are associated with the image of cancer," says Zhu.
Around one in 10 older and newer brands made direct claims about the effectiveness of e-cigarettes in helping smokers quit.
Industry claims about the devices are unsupported by evidence to date, however, according to a major scientific review of research on e-cigarettes published last month in the American Heart Association's journal Circulation.
UC San Francisco scientists, who analysed 84 research studies on e-cigarettes and other related scientific materials, found that e-cigarette use is associated with significantly lower odds of quitting cigarettes.
Based on the results of five population-based studies of smokers, they found that smokers who used e-cigarettes were about a third less likely to quit smoking than those who did not use the device.
They also found that while the data are still limited, e-cigarette emissions "are not merely 'harmless water vapour' as is frequently claimed, and can be a source of indoor air pollution".
"E-cigarettes do not burn or smoulder the way conventional cigarettes do, so they do not emit side-stream smoke. However, bystanders are exposed to aerosol exhaled by the user," the study noted.
Nicotine and toxins have been measured in that aerosol, such as formaldehyde, acetaldehyde, acetic acid, though at lower levels compared to conventional cigarette emissions.
The long-term biological effects of its use are still unknown, the authors added, concluding that the product should be prohibited wherever tobacco cigarettes are prohibited and should be subject to the same marketing restrictions as conventional cigarettes.