The UK medicines regulator has approved a brand of e-cigarette to be marketed as an aid to help people stop smoking.
The decision means e-Voke, produced by British American Tobacco, could be prescribed on the NHS.
Public Health England says e-cigarettes are far less harmful than tobacco and help smokers quit.
But some experts, including the British Medical Association, say the benefits and harms are not yet known since e-cigarettes are still relatively new.
The Royal College of GPs said doctors would be reluctant to hand them out to patients without clear merits.
Around 10m adults – one in five – in the UK smoke cigarettes.
Many of these would like to or are actively trying to kick the habit and an increasing number are turning to e-cigarettes, the NHS says.
In the year up to April 2015, two out of three people who used e-cigarettes in combination with the NHS stop smoking service managed to successfully quit.
Prof Kevin Fenton, National Director of Health and Wellbeing, Public Health England, says e-cigarettes have become the most popular quitting aid in England.
And he thinks more people should benefit.
“Public Health England wants to see a choice of safe and effective replacements for smoking that smokers themselves want to use,” he said.
But Dr Tim Ballard of the Royal College of GPs said it would be unreasonable for the NHS to be asked to fund lifestyle choices for people.
“Potentially, there may be a place for the prescription of e-Voke as part of a smoking cessation programme, but GPs would be very wary of prescribing them until there was clear evidence of their safety and of their efficacy in helping people to quit,” he said.
“At the moment there isn’t the evidence and the guidance hasn’t been written to help GPs make those decisions.”
1. On some e-cigarettes, inhalation activates the battery-powered atomiser. Other types are manually switched on
2. A heating coil inside the atomiser heats liquid nicotine contained in a cartridge
3. The mixture becomes vapour and is inhaled. Many e-cigarettes have an LED light as a cosmetic feature to simulate traditional cigarette glow.
Different brands of e-cigarettes contain different chemical concentrations.
Deborah Arnott of Action on Smoking and Heath (ASH) said: “Electronic cigarettes are a much safer alternative source of nicotine for smokers than cigarettes, but that doesn’t mean they are risk-free and we would discourage anyone who’s not a smoker from using them.
“It is good news that an electronic cigarette has received a licence from the medicines regulator, as we know that they have been effective in helping smokers quit, and the cost, as part of a quit attempt, will be far lower than treating the diseases caused by smoking.”
Another type of nicotine inhaler which closely resembles a cigarette, called Voke, was licensed in 2014 to be marketed as an aid to help people stop smoking.