One in four mice exposed to e-cigarette smoke developed lung cancer after a year – and more than half developed a pre-cursor to bladder cancer.
The researchers said the lab tests mean more studies must be done on vaping to determine its health risks for humans.
A separate group of mice were exposed to vaping smoke without nicotine – and none developed cancer.
Although the results come from vaping, researchers believe that nicotine plays a part which might have implications for other quitting devices such as patches or nicotine gum.
It comes after vaping was linked to 200 health problems – including pneumonia and heart disorders – in the UK in the past five years, despite its ‘healthy’ tag.
The illnesses were listed in 74 separate “Yellow Card” reports about e-cigarettes filed to the Medicines and Healthcare Regulatory Agency (MHRA) by the public and healthcare professionals.
Forty-nine of the complaints received since January 1 2014 were classified as serious but none of them resulted in death.
The NHS says e-cigarettes “aren’t completely risk-free” while Cancer Research UK claims: “They usually contain nicotine, which is addictive but doesn’t cause cancer.”
But New York University researchers reported nine of 40 mice they exposed to vaping smoke developed lung adenocarcinomas after 54 weeks.
Twenty-three developed bladder hyperplasia – a condition where damaged genes make cells more likely to multiply – a pre-cursor to cancer.
Meanwhile none of 20 mice that were exposed to vaping smoke without nicotine developed cancer – with exposure to that smoke leaving only one of 17 other mice with hyperplasia.
Study leader Professor Moon-shong Tang, of NYU School of Medicine, found nicotine’s cancer risk does not show up in blood tests which have been used to hail vaping as safe.
He warned vaping smoke “must be more thoroughly studied before it is deemed safe”.
Prof Tang said: “Tobacco smoke is among the most dangerous environmental agents to which humans are routinely exposed, but the potential of E-cig smoke as a threat to human health is not yet fully understood.
“Our study results in mice were not meant to be compared to human disease, but instead argue that E-cig smoke must be more thoroughly studied before it is deemed safe or marketed that way.”
The tobacco curing process can convert nicotine into cancer-causing NNN and NNK ‘nitrosamines’ which deposit in a smoker’s or9ans and blood via smoke.
Tests in 2017 found vapers had 95% less in their blood than regular smokers – causing experts to publicly claim switching to vaping could save millions of lives.
But Prof Tang said his new study showed all mammalian cells contain ions that react directly with nicotine to form NNK.
As this process happens within cells, he said the findings suggested the cancer risk of vaping was not captured in the blood tests.
Study author Dr Herbert Lepor said: “Our next step in this line of work will be to expand the number of mice studied, to shorten and prolong E-cigarette exposure time, and to further investigate the genetic changes caused by E-cigarette smoke.”
The study has been published Proceedings of National Academy of Sciences (PNAS).