Even after puffing on just one electronic cigarette with nicotine, healthy non-smokers were found to have a biological marker known to increase the risk of heart disease in tobacco users, according to a new study. The research, published in Journal of the American Heart Association, shows that nicotine is not harmless, as many people believe. It can affect a smoker’s health in more than one way, and not just by triggering addiction.

“There is this old saying that people smoke for the nicotine, cause it’s so addictive, but they die from the tar,” says study co-author Holly Middlekauff, a cardiologist at the University of California, Los Angeles. This belief that nicotine is generally safe led to nicotine-replacement therapies in the 1970s — like nicotine patches and gums — to help people quit. Today, electronic cigarettes have become popular with people addicted to cigarettes, who view vaping as less harmful than smoking.

The use of e-cigarettes, however, is fairly new, so its long-term effects are still unknown. And the scientific community has been divided about e-cig safety. Last year, a prestigious medical group in the UK recommended that tobacco smokers switch to e-cigs to quit, but the US Centers for Disease Control and Prevention said there’s no evidence that e-cigs work as a smoking cessation tool. Meanwhile, the body of research on health risk is slowly growing: a 2015 study found that e-cig vapor damages the immune system of mice. Last year, a study found that e-cigs produce 31 harmful chemicals, including some that are believed to cause cancer.

Another study, conducted by Middlekauff that was published earlier this year, showed that people who use e-cigs almost every day have biological markers known to increase the risk of heart disease in tobacco users. These included an increase in adrenaline levels in the heart, which can predispose smokers to bad heart rhythms, heart attacks, and sudden death, as well as increased oxidative stress, an imbalance in the body’s ability to defend itself against the damaging action of free radicals. Oxidative stress can lead to changes in blood fats and lead to arteriosclerosis.

That study, however, didn’t show what exactly was causing those changes. E-cigarettes can have different flavoring and solvents, as well as nicotine. So to identify the culprit, Middlekauff brought 33 healthy non-smokers and non-vapers into the lab. On three different days, one month apart, the participants were asked to puff on three different kinds of e-cigarettes for 30 minutes: one with nicotine, one without nicotine, and a sham e-cig that was empty.

The researchers did blood tests and measured the subjects’ heart rhythms, and found that the participants had high levels of adrenaline in their hearts after they smoked the e-cig with nicotine, but not after they puffed on the e-cigarette without nicotine or the empty e-cig. The researchers didn’t find increased oxidative stress, but Middlekauff says they also only looked at one marker, and more tests should be done to confirm that finding. “I think that question is still wide open,” she says.

The study has some limitations: it is small, which makes it less reliable. It also only shows that vaping affects the markers that are known risk factors, it doesn’t show that vaping is connected to actual heart disease. The only way to find that out is to do longer-term studies on e-cig users.

Still, the study “challenges the belief that nicotine is benign and without significant risk,” Middlekauff says. It also adds to our understanding of the potential health effects of e-cigarettes, which people should consider when deciding whether or not to pick up vaping. For people who smoke regular cigarettes, switching to e-cigarettes is better, because e-cigs have “orders of magnitude” less carcinogens than tobacco cigarettes, Middlekauff says. But for non-smokers, the story is different.

“If you don’t smoke at all, I would strongly advice not to start smoking electronic cigarettes,” she says. “They’re not harmless.”