(CNN) When combined with one-on-one behavior therapy, e-cigarettes are more effective in helping people quit smoking than traditional nicotine-replacement products such as patches and gum, according to a new randomized study of British adults published Wednesday in the New England Journal of Medicine.

"E-cigarettes were almost twice as effective as the 'gold standard' combination of nicotine replacement products," lead researcher Peter Hajek from Queen Mary University of London said in a statement.

"Health professionals have been reluctant to recommend their [e-cigarette] use because of the lack of clear evidence from randomized controlled trials. This is now likely to change."

But US researchers urge caution, saying the study's results are not conclusive or easy to generalize. They also point to the unknown long-term health effects of e-cigarette vapor on the body, as well as dangers of possibly encouraging e-cigarette use among teens.

Positive response in the UK

The study's results have already been widely embraced by experts in the UK, many of whom agree that e-cigarettes should be included in adult anti-smoking efforts.

"This study should reassure policymakers and health professionals -- mainly beyond the UK -- who have until now been hesitant to recommend e-cigarettes for smoking cessation on the basis that there was a lack of high-quality trial evidence," said Jamie Brown, who serves as deputy director of the Tobacco and Alcohol Research Group at University College London.

"We need to help as many people as possible to give themselves the best chance of success by using aids like e-cigarettes each time they try to stop smoking," Paul Aveyard, a professor of behavioral medicine at the University of Oxford, said in a response to the study.

But American researchers and policy makers are not convinced.

"The US Food and Drug Administration has not found any e-cigarette to be safe and effective in helping smokers quit," said Jennifer Hobbs Folkenroth, who is the American Lung Association's senior director of tobacco. "We only support methods that are FDA approved and regulated."

FDA Commissioner Dr. Scott Gottlieb recently announced a crackdown on what he called an "epidemic" of e-cigarette use among teenagers.

"E-cigarettes have volatile organic compounds, cancer causing chemicals, and flavorings that have been linked to lung disease," said Borelli, who directs the Center for Behavioral Science Research at Boston University . "Let's not say e-cigarettes are a magic pill to help people quit smoking without having all of the information."

Long term, randomized study

The multi-center trial looked at almost 900 smokers in the UK who were randomly assigned to two groups. One received a traditional nicotine replacement of their choice, such as patches, gum, lozenges, sprays, inhalators or a combination of products for three months. The other group was given an e-cigarette starter pack and encouraged to buy future supplies choosing their preference of e-juice strengths and flavors.

Both groups received one-on-one behavioral counseling each week for four weeks and were biochemically tested at the end of the year to assure they had stopped smoking.

Results showed that the one-year abstinence rate was 18% (79 people) in the e-cigarette group versus about 10% (44 people) in the traditional nicotine-replacement group. The e-cig users were more likely to have not smoked traditional cigarettes, had less cough and phlegm production at the end of a year and had less severe urges to smoke than those on replacement patches and other forms of nicotine replacement.

"This study is of huge significance," said Robert West, a professor of health psychology and director of tobacco studies at University College London. "It provides the clearest indication yet that e-cigarettes are probably more effective than products such as nicotine gum and patches."

However, of the 79 people who used e-cigarettes to quit smoking traditional cigarettes by the end of the year, 63 were still using e-cigarettes. In comparison, of the 44 people who quit smoking while using patches, gum and other types of traditional nicotine replacements, only four were still using those products at the end of that year.

Critics say those results aren't particularly positive. "Switching to e-cigarettes does not mean quitting," said Folkenroth of the American Lung Association. "Quitting means truly ending the addiction to nicotine, which is very difficult."

First randomized study to use 'modern' e-cigs

Past research using e-cigarettes as adult smoking cessation aids has been inconclusive. Many were observational and did not objectively measure a smoker's success at quitting. Others used older, first generation e-cigarettes that were poor substitutes for nicotine delivery.

Today's 400-plus e-cigarette brands are small, unobtrusive and easier to use than the older types. They also come in hundreds of flavors and are much more powerful in delivering nicotine to ease a smoker's cravings. In fact, researchers worry that e-cigarettes can be more nicotine-laden than traditional cigarettes.

For example, one Juul pod -- a cartridge of nicotine-rich liquid that users plug into the dominant e-cig brand -- contains the same amount of nicotine as a full pack of cigarettes.

One of the strengths of the UK study, said lead researcher Hajek, was that it was the first to use and test the "efficacy of modern e-cigarettes in helping smokers quit."

Borrelli agrees the study had numerous strengths: "They biochemically verified smoking outcomes. They did rigorous data analysis, they had long-term outcomes. They let people choose their e-liquid, they let people choose their nicotine replacement therapy. And it was conducted in the real world."

However, it's not clear if these findings would apply to other e-cigarette devices, or outside of the context of a well-controlled study and without behavioral counseling, said Scott Weaver, a smoking cessation researcher and Georgia State University assistant professor.

It's also unclear if the results would "generalize to other populations of smokers, particularly US smokers," Weaver said.

Trying to compare US and UK outcomes in smoking cessation are like "comparing apples and oranges," said Folkenroth, pointing to a 2018 letter to Congress by the American Medical Association and several other medical organizations explicitly stating that "claims in the UK do not really apply to the US market."

The medical organizations' letter pointed to the more strict European tobacco regulatory environment, "including advertising restrictions and health warnings on e-cigarettes," which are not fully in place in the United States. In addition, the letter said that research has not yet proven the ability of e-cigarettes to help smokers to quit cigarettes.

Another big concern is the worrisome rise in e-cigarette use among US youth -- more than 3.6 million middle school and high school students currently vape . The UK doesn't have near the number of teen users; in fact, the American Lung Association estimates that less than 6% of UK youth use e-cigs versus more than 20% in the US.

Add to that the fact that youth who use e-cigarettes have been shown to be more likely to start smoking traditional cigarettes, sometimes along with e-cigarettes.

"There's concern that the adolescent brain may be more vulnerable to the addictive effects of nicotine," said Adam Leventhal, director of the Health, Emotion, and Addiction Laboratory at the University of Southern California.

"The circuits underlying pleasure and the pursuit of novel, enjoyable experiences develop much faster than the circuits that promote decision making, impulse control and rational thinking."

Smoking cessation tips

In the United States, experts are not recommending the use of e-cigarettes to adults who are trying to kick the habit.

Instead, the American Lung Association focuses on seven FDA approved medications: the nicotine patch, gum, lozenge, inhaler, nasal spray, Varenicline (known as Chantix) and bupropion (known as Wellbutrin), Folkenroth said, "which, in addition to behavioral counseling and therapy, packs a one-two punch and is most effective in successfully helping folks to quit."

Georgia State University's Weaver said that people who have tried and failed using the approved methods "might find that later generation e-cigarettes, particularly when used daily in conjunction with a quit plan and weekly, one-on-one behavioral counseling, may improve their chances of successfully quitting."

Behavior therapy, Folkenroth stresses, must also last much longer than the four weeks provided in the UK trial. It should be weekly, one-on-one and last well past the quit date to avoid backslips.

Get CNN Health's weekly newsletter Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

Overall, US experts urge caution until further research clarifies the long-term effects of e-cigarettes on the human body and the addictive mind.

"We've come so far in tackling smoking with proven methods that have years of research behind them on their long-term benefits and side effects," Borrelli said. "Could we now start regressing with e-cigarettes, and renormalize addiction?"