Habitual e-cigarette use has been found to increase risk factors linked to negative cardiovascular effects, according to a study published in the journal JAMA Cardiology.

Most e-cigarettes, or electronic cigarettes, contain nicotine, the Centers for Disease Control and Prevention notes. Yet unlike cigarettes, e-cigarettes don’t contain tobacco.

The use of e-cigarettes has skyrocketed since they were first marketed in the United States in 2006, especially among young people. And e-cigarettes are often touted as being a safer alternative to tobacco cigarettes, which are the leading preventable cause of death in America.

But the new JAMA study suggests e-cigarettes are not harmless and can have negative effects on your heart, if you’re a habitual user.

In the JAMA Cardiology study, researchers from the University of California, Los Angeles found that healthy habitual e-cigarette users were more likely than the healthy non-smoking control participants to have increased cardiac sympathetic activity and increased oxidative stress. Both of these are known to be increased among combustible tobacco smokers and are well established cardiovascular risk predictors, said Dr. Rami Doukky, Chief of Cardiology at Cook County Health. Dr. Doukky was not part of the study.

“The study showed that habitual e-cigarette users displayed some disturbance in their heart rate variability that indicate sympathetic predominance, a risk marker for cardiac mortality. What was more surprising in the study is that e-cigarette users had evidence of increased susceptibility to LDL (harmful) cholesterol oxidation, which is an indicator of increased oxidative stress. These biochemical changes are usually associated with increased risk for coronary atherosclerosis, the disease that causes heart attack and sudden death related to heart disease,” he said.

And while additional research needs to be done, Dr. Doukky said this study is an “eye-opener” that e-cigarettes may not be as safe as initially thought.

“Although e-cigarettes are perceived as a “harmless” form of nicotine consumption, we caution that we do not fully understand the potential adverse effects of vaporized nicotine or other additives used in e-cigarettes. This study is an eye opener that e-cigarettes are not innocuous and are associated with physiological and biochemical effects that need to be fully investigated,” he said. “And although e-cigarettes are probably safer that combustible tobacco use, the perception that e-cigarettes are a “safe” alternative to tobacco smoking is not founded on evidence.”

Dr. Doukky added that “the widespread use of e-cigarettes may increase acceptance of smoking and increase nicotine addiction in society which may diminish the significant gains achieved by reducing tobacco use in western societies in recent decades.”