A new study finds e-cigarettes are associated with significantly less quitting among smokers, CBS News reports. Adult smokers who used e-cigarettes were 28 percent less likely to stop smoking regular cigarettes, researchers found.

The findings, published in The Lancet Respiratory Medicine, are the largest to date to address the question of whether e-cigarettes help smokers quit.

“E-cigarettes should not be recommended as effective smoking cessation aids until there is evidence that, as promoted and used, they assist smoking cessation,” said lead researcher Dr. Sara Kalkhoran, who was at the University of California, San Francisco (UCSF) when the research was conducted. She is now at Massachusetts General Hospital and Harvard Medical School.

The researchers reviewed 38 studies that evaluated the link between e-cigarette use and smoking cessation among adult smokers. Of those, they chose 20 studies that had control groups of smokers who did not use e-cigarettes, and combined the results.

Last year the U.S. Preventive Services Task Force concluded that there was insufficient evidence to recommend that e-cigarettes be used to help adults quit smoking. No e-cigarette manufacturers have submitted an application to the Food and Drug Administration to approve the devices for smoking cessation, the article notes.

“The irony is that quitting smoking is one of the main reasons both adults and kids use e-cigarettes, but the overall effect is less, not more, quitting,” co-author Stanton A. Glantz, PhD, Director of the UCSF Center for Tobacco Control Research and Education, said in a statement. “While there is no question that a puff on an e-cigarette is less dangerous than a puff on a conventional cigarette, the most dangerous thing about e-cigarettes is that they keep people smoking conventional cigarettes.”