E-cigarettes may not be as harmless as many people think. A new study suggests they significantly increase the risk for heart attack.

Combined data from two national surveys of more than 69,000 people 18 and older carried out in 2014 and 2016 showed that compared with people who never used e-cigarettes, daily users almost doubled their risk for heart attack.

Cigarette smoking alone nearly triples the risk for heart attack. But more than 66 percent of e-cigarette users also smoked cigarettes, and in those people the risk of heart attack was nearly five times the risk of nonsmokers.

The study was in The American Journal of Preventive Medicine.

The authors acknowledge that the observational design of the study identifies only associations, not cause and effect, and that the data relies on self reports. Moreover, there were variables such as a family history that were not included in the analysis.

Still, the lead author, Stanton A. Glantz, a professor of medicine at the University of California, San Francisco, said that switching from cigarettes to e-cigarettes is probably unhelpful.

“If you switch,” he said, “it’s almost the same as continuing to smoke. You have the residual risk of being a smoker compounded by the risk of being an e-cigarette user. The way to get rid of the risk is to stop.”

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