On Thursday, US surgeon general Vivek Murthy recommended that electronic cigarettes be incorporated into existing tax and marketing policies designed to prevent young people from smoking traditional cigarettes.

In a new report, Dr. Murthy expressed concern about the negative health consequences of e-cigarettes, which have become the most commonly used tobacco product among young people. It is the first time Murthy has commented on the subject since vaping reached peak popularity last year. Today, 5 percent of middle school students and 16 percent of high school students say they have used e-cigarettes in the past month, according to the Centers for Disease Control and Prevention (CDC).

"My concern is e-cigarettes have the potential to create a whole new generation of kids who are addicted to nicotine," Murthy told The Associated Press. "If that leads to the use of other tobacco-related products, then we are going to be moving backward instead of forward."

As e-cigarettes become increasingly popular with teens, companies are spending millions to market them: nearly seven in 10 American teenagers saw an e-cigarette advertisement in 2014, one study found. By evoking images of sex and rebellion, the CDC suggests, these ads look a lot like the ones that got yesterday’s teens hooked on traditional cigarettes.

The debate over how much of an effect such messages have on teenagers "is a big one," as The Christian Science Monitor reported in January. "The prevailing logic, however, is if ads had no impact, the companies would not invest in them," as Lucy Schouten wrote for the Monitor at the time.

While the causal relationships between the two are still unclear, some studies do suggest teenagers who vape are more likely to become traditional cigarette smokers.

In a study published by the American Medical Association in November, researchers found that teens who regularly smoke e-cigarettes were far more likely to pick up a traditional smoking habit than their peers were. That notion seems to have taken root at the CDC, as well as the FDA, which banned the sale of e-cigarettes to minors in August.

Still, researchers have yet to find conclusive evidence that e-cigarette use leads to traditional cigarette smoking. In fact, a recent report by the Substance Abuse and Mental Health Services Administration (SAMHSA) found that underage cigarette use is declining, although it did not ask respondents about e-cigarette use.

In 2015, just over 4 percent of teens aged 12 to 17 said they had recently smoked a cigarette. That’s down nearly 10 percent from 2002, when 13 percent of teens reported smoking.

Some proponents claim that "vaping," which is thought to be marginally safer than the combustible alternative, may actually keep teens from using traditional cigarettes. But that argument, just like its counter-claim, has yet to be verified.

"More studies are needed to elucidate the nature of any true causal relationship between e-cigarette and combustible tobacco product use," the Surgeon General’s report notes. “This principle supports intervention to avoid possible health risks when the potential risks remain uncertain and have been, as yet, partially defined.”

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"Your kids are not an experiment," Murthy said in a public service announcement accompanying the report.

This report contains material from Reuters and The Associated Press.