American experts are expressing caution about a UK study that shows e-cigarettes help adult smokers quit smoking at a rate almost twice as high as people who use traditional therapies such as nicotine patches or gum. Although the 18 percent success rate among the study’s e-cigarette group was low, it still higher than the 9.9 percent rate among those who used traditional nicotine replacement therapy.

Many researchers said the study published in the New England Journal of Medicine offers the first clear evidence about the effectiveness of e-cigarettes. “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.”

Doctors who have been reluctant to recommend e-cigarettes have pointed to both the lack of evidence showing they work and lack of data on the products’ long-term effects on health. The year-long study conducted at the Queen Mary University of London has found wide acceptance among experts in the UK. Many of them agree that e-cigarettes should be included in adult anti-smoking efforts.

However, some public health experts wrote in the same issue of the Journal that they believed more research is needed to back up the U.K. study. Professors from the Boston University School of Medicine wrote an editorial that accompanied the study to urge caution when recommending e-cigarettes. They said doctors should only recommend e-cigarettes when an FDA-approved treatment has not worked for their patients.

The U.S. Food and Drug Administration (FDA) has not approved e-cigarettes as a stop-smoking treatment, and American experts have been particularly concerned about marketing flavored e-cigarettes to teens. The American Lung Association estimates that more than 20 percent of U.S. youth currently vape compared to fewer than 6 percent of UK youth. More than 3.6 million middle school and high school students use e-cigarettes.

Vaping increased almost 80 percent among high schoolers and 50 percent among middle schoolers from 2017 to 2018, according to the FDA. FDA Commissioner Dr. Scott Gottlieb cited those statistics last year when he announced a crackdown against flavored nicotine products that he said have propelled an “epidemic” of e-cigarette use among teenagers.

“While e-cigarettes are ‘safer’ than traditional cigarettes, they are not without risks,” Boston University professors Belinda Borrelli and George O’Connor said in a statement. They also pointed to the long-term effect of using e-cigarettes as a smoking cessation treatment. At the end of one year in the U.K. study, 80 percent of people in the e-cigarette group had stopped smoking traditional tobacco cigarettes but were still using e-cigarettes.

They echo the study’s authors, who said the finding “can be seen as problematic if e-cigarette use for a year signals ongoing long-term use, which may pose as-yet unknown health risks.”

The study was conducted with 886 people who were attending the U.K. National Health Service’s stop-smoking services. Participants were divided into two groups and allowed to either use an e-cigarette or choose a nicotine-replacement product such as nicotine patches and gums.

Read More: E-Cigarettes Are Effective at Helping Smokers Quit, a Study Says and UK study shows e-cigarettes help adult smokers quit, but US experts urge caution