Although e-cigarettes are often marketed as devices to help smokers quit, a new study finds they do little to help smokers kick the habit.

The study, done by UCSF and published in The Lancet Respiratory Medicine, finds that smokers who also vape are 28% less likely to stop smoking cigarettes. According to UCSF, the study reviewed and analyzed the data from 38 other studies and is the largest study to date.

"E-cigarettes should not be recommended as effective smoking cessation aids until there is evidence that, as promoted and used, they assist smoking cessation," Sara Kalkhoran, MD, a clinical fellow at the UCSF School of Medicine, said in a release.

"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," says Stanton A. Glantz, PhD, a co-author on the study and director of the school's Center for Tobacco Control Research and Education.

Last year, the U.S. Preventive Services Task Force found there was no evidence to recommend e-cigarettes, or vaping pens as they are also known, to help smokers quit.

Although e-cigarettes are often promoted by manufacturers as a way to get nicotine fixes where smoking is prohibited, smoking laws and bans across the country often include e-cigarettes. Over 430 cities have laws that ban e-cigarettes where smoking is also prohibited, including San Francisco.