Now, a new report suggests that for the first time in the United Kingdom, more than half of the country's electronic cigarette users, or "vapers", have since given up smoking tobacco.
Professor Ann McNeill, Professor of Tobacco Addiction at King's College London, said in a press release Monday that the most common reasons given by e-cigarette users for switching from tobacco were to help them stop smoking entirely and to save money:
"This year's ASH survey finds that around 1.5 million vapers are ex-smokers, for the first time a larger number than those who continue to smoke.
"This is encouraging news as we know that vapers who continue to smoke continue to be exposed to cancer-causing substances. The message for the 1.3 million vapers who still smoke is that they need to go further and switch completely," she said.
Deborah Arnott, chief executive of public health charity ASH, said the study was encouraging but many still overestimated the risks associated with e-cigarettes.
"The rapid growth in e-cigarette use has come to an end while over a third of smokers have still never tried e-cigarettes, saying the main reasons are concerns about the safety and addictiveness of e-cigarettes.
It's very important smokers realize that vaping is much, much less harmful than smoking," she added.
The e-cigarette industry was estimated in a December report to be worth $32bn (£25bn) by the year 2021.
And as Washington clamps down on e-cigarettes, one drug abuse researcher has said U.S. lawmakers must recognize that vaping can help smokers quit.
"As a harm-reduction tool, e-cigarettes should be available, even promoted, to current smokers as an alternative to traditional cigarettes," said Carrie Wade in an opinion piece published on "The Hill" website last Wednesday.
"History has proven that abstinence approaches do not work: Teen pregnancy and sexually transmitted infection rates did not decline in states that emphasized an abstinence-only sex education curriculum and criminalization of heroin use did not stop overdose deaths," she added.
Wade wrote that many e-cigarette manufacturers and retailers will now struggle to continue to do business as they are forced to go through a "costly" pre-market tobacco application (PMTA) process for every product they sell.
FDA figures estimate cigarette smoking is responsible for more than 480,000 deaths per year in the United States.