E-cigarette flavorings may pose heart risk, study finds

What they're saying: "Until now, we had no data about how these e-liquids affect human endothelial cells," said study co-author and Stanford Cardiovascular Institute director Joseph Wu.

"This study clearly shows that e-cigarettes are not a safe alternative to traditional cigarettes. When we exposed the cells to six different flavors of e-liquid with varying levels of nicotine, we saw significant damage. The cells were less viable in culture, and they began to exhibit multiple symptoms of dysfunction."

The other side: A January study found e-cigarettes are an effective way for smokers to quit smoking — twice as effective as other nicotine products like gum and patches.

Go deeper: Podcast: The war on vaping