A study that shows nicotine contributes to smokers’ higher risk of developing heart disease suggests the nicotine in e-cigarettes is not necessarily safe for the heart, CNN reports.

The study found human and rat heart cells exposed to nicotine showed changes after only six hours. A type of cellular drill formed and ate through tissue. The result was the formation of plaque, which forms in heart disease, the researchers reported this week at the American Society of Cell Biology.

“These findings suggest that e-cigarettes, the battery-powered devices that deliver nicotine in steam without the carcinogenic agents of tobacco smoke, may not significantly reduce smokers’ risk for heart disease,” researcher Chi-Ming Hai, PhD, of Brown University, said in a news release.

Some studies have suggested e-cigarettes may help smokers quit. A study published this week concluded people who use e-cigarettes indoors may be exposing the people around them to nicotine.