Vaping flavours poisons the immune system - raising the risk of infections and life-threatening diseases, according to new research.
A study suggests cinnamon, vanilla and buttery e-cigarette flavours are among the most dangerous - and mixing them is more damaging than vaping only one.
Flavouring chemicals and liquids used in the battery-powered devices are toxic to the white cells in the blood, warn scientists.
Some have "alluring names" such as 'candy', 'cake', 'cinnamon roll' and 'mystery mix' that attract teenage vapers.
But exposure can cause significant inflammation to monocytes - the biggest type of white blood cell in the immune system that combat bacteria and viruses.
Originally formed in the bone marrow, they are released into our blood and tissues. When certain germs enter the body they quickly rush to the site for attack.
Inflammation is linked to all major illnesses including cancer, heart disease, diabetes and even dementia.
The study, published in the journal Frontiers in Physiology, adds to growing evidence on the harmful health effects of e-cigarettes.
The use of e-cigs has exploded in the past decade. In the US alone more than 500 brands with almost 8,000 uniquely flavoured e-juices are available to consumers.
Vaping exposes the lungs to flavouring chemicals when the e-liquids are heated and inhaled.
Since these molecules are considered safe to eat the devices are promoted as a healthier alternative to traditional cigarettes.
But the health effects of inhaling them are not well understood. So in lab experiments the researchers directly exposed monocytic blood cells to e-liquids.
It led to higher production of two well-established biomarkers - or chemical clues - for inflammation and tissue damage triggered by oxidative stress.
And many of the flavouring chemicals caused significant cell death - with some being worse than others.
First author Dr Thivanka Muthumalage, of Rochester University, said the compounds tested may be safe for ingestion - but not for inhalation.
He said: "Cinnamon, vanilla and butter flavouring chemicals were the most toxic but our research showed mixing flavours of e-liquids caused by far the most toxicity to white blood cells."
Previous studies have shown e-cig flavours cause inflammatory and oxidative stress responses in lung cells.
Users also have increased levels of oxidative stress markers in the blood compared to non-smokers.
The new findings extend this to assess the effects of commonly used flavourings - as well as e-liquids without nicotine - directly on monocytes.
Senior author Dr Irfan Rahman hopes the new data will provide insights into understanding the harmful effects of flavoured e-juices.
He said: "Currently these are not regulated and alluring flavour names such as candy, cake, cinnamon roll and mystery mix, attract young vapers.
"Our scientific findings show e-liquid flavors can - and should - be regulated and e-juice bottles must have a descriptive listing of all ingredients. We urge regulatory agencies to act to protect public health."
The team plans to undertake further research to simulate live vaping by exposing cells to e-liquid aerosols in an air-liquid interface system.
They also call for further long-term human studies to assess the harmful effects of e-cigarettes.
Earlier this week a study of mice reported by another US team suggested e-cig users face an increased risk of cancer and heart disease compared to non-smokers.