Scientists have warned flavoured e-cigarette liquids can be "deadly" and "lead to strokes and heart attacks".
A study by researchers from Boston University has found the increasingly-popular vapes can actually damage cells lining in the heart and blood vessels.
Such damage can then lead to heart disease and heart attacks.
The US tests found nine particular flavours posed a risk, which are: menthol, vanilla, cinnamon, clove, strawberry, banana, burnt, butter and spicy cooling.
The test found additives in these flavours slowed blood flow and increase inflammation.
They’re often portrayed as ‘healthier’ alternatives to cigarettes, but scientists have warned about the dangers of e-cigarettes.
Dr Jessica Fetterman, who led the study, said: “Increased inflammation and a loss of nitric oxide are some of the first changes to occur leading up to cardiovascular disease and events like heart attacks and stroke, so they are considered early predictors of heart disease.
“Our findings suggest that these flavoring additives may have serious health consequences.
“Our work and prior research have provided evidence that flavorings induce toxicity in the lung and cardiovascular systems.
“Flavorings are also a driver of youth tobacco use and sustained tobacco use among smokers.”
Hope for new vaping regulations
The researchers hope their findings will lead to new regulations to prevent access, sales and marketing of e-cigarettes to youth.
Cinnamon-flavoured e-cigarettes may damage the lungs, a study has found.
The chemical that gives the spice its characteristic smell and taste has a similar effect on lung cells as toxins in cigarette smoke, say researchers.
Scientists in the US carried out tests in which they exposed human airway cells to cinnamon-flavoured liquid and spray from one of the latest e-cigarettes.
They found that the spice chemical cinnamaldehyde impaired the movement of tiny hair-like cilia that project from bronchial cells.
In healthy lungs, the beating cilia perform the vital function of clearing away mucus and dirt, along with bacteria and viruses.
'Similar to their toxic tobacco counterparts'
Lead scientist Dr Phillip Clapp, from the University of North Carolina, said: "Our data suggest that when used in e-cigarettes cinnamaldehyde, like toxic aldehydes in cigarette smoke, significantly disrupts normal cell physiology in ways that may have implications for the development and exacerbation of respiratory disease.
"Our finding that cinnamaldehyde impairs normal airway cilia motility is significant because it demonstrates that a common, food-safe flavouring agent, in the context of e-cigarette use, is capable of dysregulating a critical anti-bacterial defence system in the lungs."
Reactive aldehyde chemicals in cigarette smoke are known to cause lung inflammation and increase susceptibility to bacterial and viral infection.
E-cigarettes also contain aldehydes as flavouring agents, many of which are structurally similar to their toxic tobacco counterparts, said Dr Clapp.
He added: "E-cigarette emissions contain chemicals that have not been evaluated for inhalation toxicities.
"The inhalation of flavouring agents, which are frequently reactive aldehydes, poses a significant unknown in regards to the potential health risks of e-cigarette use."
In addition, the flavouring agents were often used in "exceedingly high concentrations" in e-cigarettes, he said.
The findings were presented at the American Thoracic Society's annual meeting in San Diego.