When a new product comes on market, it’s not always immediately clear whether or not it’s safe. This has been the case for quite a few chemicals that have ultimately been banned after being around for many years. It is also the reason a new paper from the National Academy of Sciences, Engineering and Medicine (NASEM), a nonprofit, non-governmental organization, takes a look at the evidence for and against the use of e-cigarettes’ in different circumstances to see if they are the safe alternative to smoking that they’ve been claimed to be.

The bottom line is that, based on what’s known so far, e-cigarettes are certainly "healthier" than combustible (old-fashioned) cigarettes. Since they don’t burn tobacco, they lack many of the chemicals in regular cigarette smoke, dozens of which cause cancer. So switching to e-cigarettes in an effort to quit regular cigarettes is almost certainly a good thing.

Lack of evidence does not mean evidence of no risk. It may just be too early to tell.

But they’re not totally innocuous — they contain some harmful chemicals of their own, so they shouldn’t be thought of as a risk-free alternative.

Another concern is a psychological one, especially for teens who are more likely to start smoking e-cigs these days than regular ones. The worry is that e-cigarettes will become a gateway to smoking regular cigarettes in the future. There had been some evidence that smoking e-cigarettes made teens more likely to try regular cigarettes down the road, but it wasn’t clear whether these individuals would actually become regular smokers.

So it's at least partly a question of a person's smoking history. Access to e-cigarettes may encourage teens to begin smoking, but those who have been smoking cigarettes for years may find e-cigs a helpful way to quit.

“E-cigarettes have the potential for enormous benefit if they help smokers quit,” said study's author, Nancy Rigotti, Director of the Tobacco Research and Treatment Center at Massachusetts General Hospital, in a statement. “This benefit must be balanced against potential harm if e-cigarettes entice youths who would not otherwise have become cigarette smokers to try e-cigarettes, become addicted to nicotine and then switch to combustible cigarettes.”

And if people do a mix of e-cigarettes and combustible cigarettes, this of course also poses a health problem, since even light smokers are at increased risk for heart disease and cancer.

Research into e-cigarettes is in its infancy. Since the products are still relatively new, we don’t yet have the kind of long-term studies on people who have smoked only e-cigarettes for many years to be able to draw firm conclusions. As Rigotti says, “lack of evidence does not equal evidence of no risk.” It may just be too early to tell.

But if you’re trying to quit smoking, switching to e-cigarettes is almost certainly a step in the right direction. And doctors should probably relay that idea to their patients.

“I tell patients that using e-cigarettes is less harmful than continuing to smoke cigarettes,” says Rigotti, “but because e-cigarettes are so new, I caution them that many questions about their long-term safety remain unanswered.” She generally recommends switching completely to e-cigs, rather than only partially and urges patients to plan to quit e-cigarettes at some point, too, given the unanswered safety questions.

The study is published in National Academy of Sciences, Engineering and Medicine (NASEM).