MPs are to carry out an inquiry into e-cigarettes amid concerns there are "significant gaps" in what is known about them and how they are regulated.
The science and technology committee will look at their effectiveness as a stop-smoking tool and the impact of their growing use on health.
Nearly three million people in the UK now "vape" regularly – four times more than in 2012.
But committee chair Norman Lamb said there was mixed messaging on vaping.
The Liberal Democrat MP said: "They are seen by some as valuable tools that will reduce the number of people smoking 'conventional' cigarettes, and seen by others as 're-normalising' smoking for the younger generation.
"We want to understand where the gaps are in the evidence base, the impact of the regulations, and the implications of this growing industry on NHS costs and the UK's public finances."
The announcement comes after e-cigarettes were included in this year's Stoptober campaign – aimed at helping people stop smoking – for the first time.
The government-backed campaign, which has been running during October, now features vaping in its TV adverts.
It came after the smoking devices proved to be the most popular tool for quitting during the 2016 campaign.
But despite this, e-cigarettes are not yet officially prescribed on the NHS.
New draft guidance from the National Institute for Health and Care Excellence (NICE) does not list e-cigarettes as a recommendation to help people quit either.
But it does say patients should be told some smokers have found them helpful when they want to give up.
NICE advises that patients should be told that there "is currently little evidence on the long-term benefits or harms of these products".
The cross-party group of MPs has asked for written evidence to be submitted by 8 December.