Electronic cigarettes (ECIGs) are booming in popularity, but there’s still only limited evidence on their potential health risks, or their advertised benefits in helping people to quit smoking, according to a research review in the July/August Journal of Addiction Medicine, the official journal of the American Society of Addiction Medicine.

Based on their review, researchers write, “Very little is known about the acute and longer-term effects of ECIG use for individuals and the public health, especially given the dramatic variability in ECIG devices, liquids, and user behavior.” The work was funded by the National Institute on Drug Abuse of the National Institutes of Health and the Center for Tobacco Products of the U.S. Food and Drug Administration.

There is little or no evidence on the long-term effects of ECIG use. The authors write, “No existing studies address the extent to which the inhalation of ECIG vapor hundreds of times every day over a period of multiple years influences human health, particularly pulmonary function.”

There is some evidence that ECIGs can reduce tobacco abstinence symptoms in cigarette smokers, but it’s unclear how much this is related to nicotine delivery or to psychological effects. So far, the best available studies — two randomized clinical trials — have shown no significant difference in quit rates among in ECIG users.