A doctor from Tyneside has called for more research into "electronic" cigarettes following the death of one of his patients.
It comes after an inquest recorded an open verdict into the death of Terence Miller from Gateshead.
Mr Miller, who used large quantities of the substitutes, had suffered from a lung disease - severe lipoid pneumonia.
Dr Rob Allcock, who treated him at the Queen Elizabeth Hospital, believes this could be associated with his use.
The "e-cigarettes" are battery-powered and contain a cartridge of liquid nicotine.
This is heated and the user inhales vaporised droplets of the drug and breathes out a mist rather than smoke.
Supporters describe them as a healthy alternative to real cigarettes because their users can inhale nicotine without tar, tobacco or carbon monoxide.
Dr Allcock said that the brand Mr Miller had been using seemed to involve a mixture of nicotine and some oil.
"There's extensive literature in the medical world on damage to the lungs due to inhaling oil, which looks very similar to his disease," he said.
"There's some limited research merely mentioning what the chemical composition [of "electronic" cigarettes] is, but there's no systematic research assessing the overall safety of inhaling these chemicals deep into the lungs over an extended period."
However, the Electronic Cigarette Industry Trade Association, has described the cigarettes as "far safer than ordinary ones".
It said that 100% of users were former smokers and any damage to their lungs was going to be caused by that.
It added "electronic" cigarettes had been "tested vigorously" in various scientific studies around the world.