There isn't enough evidence yet to say whether e-cigarettes are safe or effective for helping people quit smoking, new U.S. guidelines on tobacco cessation conclude.

The conclusion, issued by the U.S. Preventive Services Task Force (USPSTF) today as a part of updated recommendations on smoking cessation, adds to an emerging consensus in the medical community that e-cigarettes come with too many unknowns for them to be recommended to current smokers looking to kick the habit.

"There is not enough evidence to evaluate the effectiveness, safety or benefits and harms of using e-cigarettes to help people quit smoking," said Dr. Francisco Garcia, a task force member and researcher at the University of Arizona.

Battery-powered e-cigarettes typically feature a glowing tip and a heating element that turns liquid nicotine and other compounds into a cloud of vapor that users inhale.

While tobacco control advocates fear that e-cigarettes may give rise to a new generation of nicotine addicts who eventually transition to conventional cigarettes, some small studies have suggested the devices might benefit the health of people who already smoke.

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An international analysis of published research by the Cochrane Review in December concluded the devices could help smokers quit but said much of the existing research on e-cigarettes was thin.

The USPSTF, a government-backed independent organization that reviews medical evidence, last updated its tobacco cessation guidelines in 2009. Then, as now, the task force found too little evidence to weigh in for or against e-cigarettes as a cessation aid. These guidelines are widely used to determine whether insurance will pay for screenings and treatments and are widely followed by primary care physicians.

"The task force recommends that clinicians direct patients who smoke tobacco to other cessation interventions with established effectiveness and safety," Garcia said by email. "Many studies show that combinations of behavioral interventions or pharmacotherapies can help the most."

Carrie Patnode, lead author of the research review USPSTF used as the basis for its updated guidelines, said that to date, just two large trials assessing e-cigarettes for smoking cessation met the gold standard for medical research, i.e., randomly assigning some patients to receive the treatment and others to receive a placebo or alternatives.

One of the two trials involved about 650 smokers and found no significant difference in smoking cessation after six months for e-cigarettes containing liquid nicotine, e-cigarettes without nicotine, or nicotine patches.

The second trial followed 300 smokers for one year and found e-cigarettes containing nicotine to be slightly better than the devices without nicotine cartridges, with cessation rates of 11 percent and 4 percent, respectively.

Both of these trials were conducted using older models of e-cigarettes, one of which is no longer available, and both studies had large numbers of participants drop out before the end.

The review, along with the task force recommendations, was published today in the Annals of Internal Medicine.

One challenge that faces doctors and scientists who assess e-cigarettes is the sheer variety of options, all of which may contain different amounts of nicotine, chemicals and additives that might impact the safety and effectiveness of the devices as cessation aids.

"There is a large variation in the devices and cartridge fluids publicly available and new products are rapidly entering the market," Patnode, a research associate at Kaiser Permanente's Center for Health Research in Portland, Oregon, said by email. "It is clear that more data on both the benefits and harms of these products - from well-designed trials - is needed."