The longstanding debate over whether electronic cigarettes help stop smoking or encourage it now has its answer: They appear to do both.   

A much-talked-about study published earlier this week says that e-cigarettes, or e-cigs, may be a safe and useful tool for quitting smoking. But another study, out yesterday, suggests that e-cig use by teenagers may actually lead to tobacco dependence.

How can something that helps adults kick an unhealthy habit increase the likelihood that teens will pick it up? Here's how to make sense of these seemingly conflicting findings. 

E-Cigs Might Help Adults Quit Smoking

The first study, published in Annals of Internal Medicine, looked at levels of nicotine and the various toxins and carcinogens typically found in tobacco smoke in 181 U.K. adults.

All were current or former smokers, and some were using e-cigs or nicotine replacement therapy (NRT) with or instead of tobacco.

The main finding—that smokers who swapped traditional cigarettes for e-cigs had much lower levels of tobacco-related toxins and carcinogens in their urine and saliva than those who stuck with tobacco—is noteworthy.

But the researchers also discovered that e-cigs supply about as much nicotine as traditional cigarettes and NRT do.  (Nicotine is highly addictive but not linked to cancer, unlike the tobacco smoke chemicals benzene and cadmium.)

So e-cigs can apparently provide enough nicotine to keep users satisfied as they wean themselves from tobacco.

That, say the study authors, bolsters claims that the electronic devices may help people loosen their dependence on tobacco.

"Our study suggests that e-cigarettes are likely to be a useful tool for quitting smoking, while at the same time dramatically reducing exposure to known smoking-related toxicants and carcinogens," says Lion Shahab, Ph.D., senior lecturer in the Department of Epidemiology and Public Health at University College London.

Other studies have explored e-cig safety and effectiveness as a tool for quitting tobacco, with varied results. But a couple of factors make this small study significant (though not the last word on the subject, since e-cigs also contain potentially problematic substances such as formaldehyde and acetaldehyde, may carry heart risks, and explode from time to time).

This study was the first to look at levels of cigarette-related substances and their by-products in the bodies of nicotine users, instead of in, say, animals or e-cig vapors.

And in the realm of e-cig research, where studies tend to be brief, it’s long-term: all the subjects had regularly used nicotine, either in regular cigarettes, e-cigs, NRT, or a combination, for at least six months.

E-Cigs Might Turn Teens Into Smokers

Several studies in recent years have suggested that e-cigs may be a gateway product to tobacco use—at least for teenagers. And yesterday's study, published in the journal Tobacco Control, adds some fuel to the fire.

Researchers surveyed 347 high school seniors about their e-cig use, then followed up with them a year later. The disturbing finding: Teens who had "vaped"—used e-cigs—in 12th grade were more than four times more likely to smoke tobacco in the following year than other adolescents.

“In this study, even kids who thought smoking was dangerous were still more likely to smoke if they used e-cigarettes," says Douglas Kamerow, M.D., a professor of clinical family medicine at Georgetown University in Washington, D.C., and a former assistant surgeon general, who was not involved in the study. “That gives us more reason to be anxious about getting kids not to vape.”

There are other reasons to keep kids from using e-cigs. Some research suggests nicotine may be more addictive for younger people and may even prime the brain for addiction to other substances. And a study out this week found that 25 percent of high schoolers who have used e-cigs have tried “dripping”—dribbling e-cig liquid onto a heated atomizer, which may expose them to higher levels of nicotine and toxins.)

How Habits Stop—and Start

So, why might e-cigs help adults quit but entice teens into using tobacco? It's likely because each group has its own reason for trying e-cigs, say experts.

“Many adults start vaping specifically as a way to stop smoking,” says Richard Miech, Ph.D., lead author of the Tobacco Control study and professor at the Institute for Social Research at University of Michigan.

But the majority of teens try vaping out of a desire to experiment and because they enjoy the flavor of e-cigs. Vaping, in turn, may desensitize them to the hazards of tobacco.

“Youth who vape may not experience any immediate, negative health consequences and come to believe that maybe smoking (tobacco) isn’t so dangerous after all,” says Miech. “Another theory is that youth who vape develop friendship networks with smokers, which increases their risk of smoking.”  

And the differences in motivation can lead to different results, notes Marvin M. Lipman, M.D., Consumer Reports' chief medical adviser: “E-cigarettes could turn out to be a two-edged sword. They may eliminate the carcinogenic aspect of cigarette smoking in adult smokers and, paradoxically, lead young e-cigarette vapers into becoming confirmed cigarette smokers.”