A new study provides some of the first evidence ever to link e-cigarette use with cardiovascular disease. The findings, published today in JAMA Cardiology, promise to fuel a debate that has perplexed both consumers and public health officials since 2006, when e-cigarettes first appeared in the U.S. marketplace.

On one side of that debate are e-cigarette proponents who say that these devices mark a positive step forward in tobacco control: E-cigarettes are presumed to be both safer and less addictive than traditional cigarettes because they contain no tobacco, no tar, and few of the related toxins that have been conclusively linked to cancer and other deadly diseases.

On the other side are critics who argue that in fact, e-cigarettes are no safer or healthier than traditional cigarettes, and who point out that they might ultimately be worse for public health: By luring new recruits with a false promise of risk-free puffing, these new devices threaten to erode a half-century's worth of public-health gains made through hard-fought, multipronged anti-smoking campaigns. 

This latest research comes down—ever so slightly—on the side of e-cigarette critics. Scientists at the University of California at Los Angeles, compared 23 "habitual" e-cigarette smokers (meaning they puffed at least once a day) to 19 nonsmokers. They found that e-cigarette smokers were more likely than nonsmokers to have abnormal heart-rate variability and higher levels of oxidative stress. Both factors are associated with an increased risk of cardiovascular disease, and both have previously been found in smokers of traditional tobacco cigarettes.

In other words, the findings suggest that e-cigarettes may cause the same types of changes in the cardiovascular system as those caused by regular cigarettes.

"It's an intriguing study," says Aruni Bhatnagar, Ph.D., a professor of medicine at the University of Louisville in Kentucky who was not involved in the research. "It adds to the growing body of evidence suggesting that e-cigarettes may have adverse cardiovascular effects."

The researchers also found that the extent of those physiological changes were directly related to the amount of e-smoking. That is, the more a person smoked the more abnormal their heart rate variability and the greater their oxidative stress ultimately was.  

To be sure, those findings come with some significant caveats. For one thing, 42 subjects makes for a relatively small study. For another, the researchers didn't look directly at the incidence of heart disease; they only examined secondary endpoints known to be associated with the condition. More and bigger (and more direct) studies will be needed before anyone can say conclusively what risks e-cigarettes pose to the cardiovascular system, and whether those risks are anywhere near as dire as the ones posed by traditional cigarettes. 

Studies of that kind will take a long time to produce. But even without them, it's increasingly clear that e-cigarettes come with health risks of their own. Until researchers figure out the size and scope of those risks, the best and safest option will be to not smoke at all.