Experts discovered blood cells exposed to e-liquid start to exhibit “significantly increased levels” of DNA damage and cell death.
It was found that the level of damage is also dependent on the flavour of the vape liquid.
Cinnamon and menthol have been found to be “particularly harmful” to heart health.
Study senior author Professor Joseph Wu, of Stanford University School of Medicine, concluded e-cigs are “not a safe alternative to traditional cigarettes”.
Scientists found that many liquids were “moderately toxic” to cells.
“The public has this notion that e-cigarettes are safe.”
Professor Wu said: “Until now, we had no data about how these e-liquids affect human endothelial cells.
“This study clearly shows that e-cigarettes are not a safe alternative to traditional cigarettes.
“When we exposed the cells to six different flavours of e-liquid with varying levels of nicotine, we saw significant damage.”
Cells, known as endothelial cells, line the surface of the blood vessels and help keep your heart healthy.
The scientists said some of the changes were observed in the absence of nicotine.
Professor Wu added: “The public has this notion that e-cigarettes are safe.
“As a result of this perception, a lot of kids pick up e-cigarette smoking.
“There’s so many kids who are smoking e-cigarettes.”
He went on: “And these kids are going to become adults.
“And these adults can become elderly patients that I as a cardiologist will take care of later on.”
The scientists warned the sweet or fruity flavours of e-liquids can make people believe vaping is harmless.
It also stems from the presence of fewer cancer-causing chemicals than combustible cigarettes.
The study was first published in the Journal of the American College of Cardiology.