A major scientific review released today suggests that e-cigs containing nicotine may help people quit smoking, in part because the devices provide a similar sensory experience. The review also found that using e-cigs for up to two years does not seem to be associated with any serious side effects.

But there are still plenty of reasons to be concerned about the devices.

For one, the authors acknowledge that their conclusions are based on less-than-perfect research. For example, of the 24 studies the authors analyzed, only two were randomized controlled trials—considered the gold standard in medical research—that followed participants for at least six months. Those two trials compared electronic cigarettes containing nicotine with a nicotine-free placebo e-cig.

The authors of the review, conducted by the independent Cochrane Library, were also unable to find enough reliable studies to determine whether using e-cigarettes to quit was more effective than using a nicotine patch—a Food and Drug Administration-approved smoking cessation device with a strong track record.

And in 2015, the U.S. Preventive Services Task Force, an independent group that advises the government on healthcare issues, concluded, "The current evidence is insufficient to recommend electronic nicotine delivery systems for tobacco cessation in adults, including pregnant women."

What’s more, the authors admit that they still can’t determine whether vaping is safe in the long run given that there simply isn’t enough long-term evidence available.

That’s hardly surprising considering that e-cigs have only been on the market for 10 years. “Keep in mind that tobacco cigarettes were initially marketed as healthful and stylish,” says Consumer Reports chief medical adviser Marvin M. Lipman, M.D. “It took years to realize the damage they cause.”

E-Cig Dangers

In addition, a number of other recent studies have raised concerns about e-cigarettes. For example, a 2016 study in the journal Environmental Science & Technology found that e-cig vapor contains 31 chemicals—many of which may cause cancer. Surprisingly, that study traced most of those chemicals back to the solvents used in e-liquids, not the chemical flavorings or other more obscure additives.

Other research also suggests that e-cigs may be hooking a whole new generation on nicotine. Early exposure to nicotine can stunt cognitive development. Nicotine dependence in later adolescence and early adulthood can lead to a lifelong nicotine habit and to higher rates of addiction.

That e-cig habit could also be luring people into smoking conventional cigarettes. Studies suggest that teens who vape are more likely to try tobacco cigarettes, too. “These devices, which are portrayed as a healthier alternative to tobacco cigarettes, could undo decades worth of progress in tobacco control,” Lipman says.

In addition, there have been numerous reports of e-cigarette batteries exploding, causing fires resulting in serious injuries.

There’s no doubt that kicking the smoking habit delivers enormous health benefits—for instance, lowering the risk of cancer, heart disease, and stroke. There’s also no question that quitting is tough. E-cigarettes may help wean smokers off cigarettes. But, Lipman says, “Until we have conclusive proof that they don’t cause harm, don’t assume that vaping is risk-free.”