More must be done to examine the impact of e-cigarettes on health, experts have said.
The call comes as new research showed that almost 30 million people around Europe have tried the battery operated products.
A new study published in the journal Tobacco Control examined data from 26,500 people across 27 European countries.
The authors of the study, from Imperial College London, the US and Greece, found that 20.3 per cent of current smokers, 4.7 per cent of ex-smokers and 1.2 per cent of never smokers had tried an e-cig at least once. This means that 29.3 million European adults have tried the products, they added.
But the researchers said that the scientific community is yet to provide information regarding the harm or efficacy of e-cigarettes.
Meanwhile a separate study, published in a special supplement of Tobacco Control, found that the market of e-cigarettes is growing rapidly.
Researchers from the US sought to identify the quantities of these products available online. They did two separate internet searches of English language websites, the first between May and August in 2012 and the second between December 2013 and January 2014.
In the period between searches there was a net increase of 10.5 new brands and 242 new flavours every month, they said.
By January 2014, they identified 466 brands with 7,764 unique flavours.
The research was published as health experts from around the globe called on the World Health Organisation (WHO) to put new controls on e-cigarettes.
In a letter to WHO director general Margaret Chan, they raised concerns about what the unregulated products actually contain.