The number of people using e-cigarettes in the UK is expected to reach a million this year but while some believe the electronic alternative to tobacco could help save hundreds of thousands of lives others think they normalise what looks like smoking and may be unsafe.
Anyone walking into a busy pub in Manchester may well be confronted with a rather shocking sight.
At one table it looks like a group of friends are smoking, but there is no smell in the air and no ashtrays on the table. What they are using are e-cigarettes.
One of the women, Steph, says the e-cigarette has helped her to stop smoking.
"I've tried patches and inhalators," she says. "They're a lot better because you feel like you're having a cigarette."
"They're a great idea," says another woman, Lisa. "You've got the health benefits from it and it does taste like a cigarette."
The e-cigarette comes in two parts.
In one end there is liquid nicotine, in the other a rechargeable battery and an atomiser. When the user sucks, the liquid nicotine is vaporised and absorbed through the mouth. What looks like smoke is largely water vapour.
Because there is no tobacco in e-cigarettes, there is no tar and it is the tar in ordinary cigarettes that kills.
The e-cigarette market is growing fast. A survey by the charity Action on Smoking and Health (ASH) suggests 700,000 people in the UK were using e-cigarettes last year.
The charity estimates that number will reach a million in 2013 and some medical experts see huge potential benefits.
"Nicotine itself is not a particularly hazardous drug," says Professor John Britton, who leads the tobacco advisory group for the Royal College of Physicians.
"It's something on a par with the effects you get from caffeine.
"If all the smokers in Britain stopped smoking cigarettes and started smoking e-cigarettes we would save 5 million deaths in people who are alive today. It's a massive potential public health prize."
There are however concerns about the safety and regulation of e-cigarettes.
They can legally be sold to children. There are few restrictions on advertising. Critics say some of the adverts glamorise something that looks like smoking.
Unlike patches and gum, e-cigarettes are not regulated like medicines. It means there are no rules for example about the purity of the nicotine in them.
So are e-cigarettes safe?
"The simple answer is we don't know," says Dr Vivienne Nathanson from the British Medical Association (BMA).
"It's going to take some time before we do know because we need to see them in use and study very carefully what the effects of e-cigarettes are."
The BMA is just one of the bodies to respond to a consultation on e-cigarettes by the Medicines and Healthcare products Regulatory Agency. The agency is deciding whether the e-cigarettes should be licensed as a medicine and more tightly regulated. The BMA thinks they should.
"I would either take them off the shelves or I would very heavily regulate them so that we know the contents of each e-cigarette were very fixed," says Dr Nathanson.
E-cigarettes are currently classed as a general consumer product and regulated by trading standards. It means they cannot contain hazardous chemicals, for example, and that the battery in them must meet EU standards.
The trade association for e-cigarettes, the Electronic Cigarette Industry Trade Association, says they make no medicinal claims for their product. It is sold merely as an alternative to ordinary cigarettes.
Attempts to classify e-cigarettes as a medicinal product have been made in Holland and Germany but the industry successfully overturned the decisions in court.
One UK based distributor, called VIP, says over stringent regulation could see them go out of business. Nonetheless Andy Whitmore, the company's marketing director, said it would "welcome regulation that ensures the product can't be sold to anyone under the age of 18".
There are many other questions. For example, should using e-cigarettes be allowed in a public place? At the offices of UK Fast - an internet storage company - employees can use them at their desk.
"It's a tricky one," says the company's chief executive officer, Lawrence Jones.
"It does look like smoking but could you stop someone from chewing a pencil or biting their nails? I don't think there's any difference between going for a caffeine break and having a nicotine break."
Other companies have banned it. But in theory electronic cigarettes can be used anywhere - on planes, trains, in hospitals.
The BMA is worried that the more people start using e-cigarettes the more it will normalise something that looks like smoking. They have called for the ban on smoking in public places to be extended to e-cigarettes.
A decision on whether the regulation of electronic cigarettes should be tightened will be made in a few weeks.