Electronic cigarettes could put smokers at risk of heart disease, a study shows. Regular cigarettes are known to cause cardiovascular diseases like heart attacks and strokes. Now, it looks like e-cigarettes might, too.

Compared to healthy non-smokers, people who use e-cigs almost every day were found to have biological markers known to increase the risk of heart disease in tobacco users, according to a study published today in the journal JAMA Cardiology. The researchers observed increased levels of adrenaline in the heart, as well as increased oxidative stress, an imbalance in the body’s ability to defend itself against the damaging action of free radicals.

A vigorous debate about e-cigs’ safety

Today’s study is part of a vigorous debate about e-cigarettes’ safety. The use of e-cigarettes, also called vaping, is a fairly new phenomenon, and the body of research on its effects is relatively thin. Despite the fact that vaping’s long-term effects are unknown, it’s become popular with people addicted to cigarettes, who view vaping as less harmful than smoking. Vaping is also increasing in popularity among teenagers. Previous research has suggested that e-cigarettes may produce carcinogens. An earlier study also linked e-cigarettes to changes in oxidative stress.

What today’s study adds is evidence that e-cigarette use has negative effects on the body, lead author Holly Middlekauff, a cardiologist at the University of California, Los Angeles tells The Verge. “My patients need to know and the public needs to know that if you don’t already smoke tobacco cigarettes, you shouldn’t start smoking e-cigarettes because they’re not harmless,” she says.

E-cigs have been on the market for about a decade, and are increasingly popular. The percentage of US adults who use e-cigs rose to 8.5 percent in 2013 from 3.3 percent in 2010. And in 2014, nearly 13 percent of adults said they tried electronic cigarettes, according to the CDC. More than 3 million American teens also vaped in 2015, a tenfold increase over four years that US Surgeon General Vivek Murthy called a public health crisis.

While popular, though, the scientific community has been divided about their safety. Last year, a prestigious medical group in the UK recommended that tobacco smokers switch to e-cigs to quit smoking, but the CDC said in a statement there’s no evidence that e-cigs work as a smoking cessation tool. Meanwhile, many in the medical community deem vaping to be a gateway to tobacco smoking, especially among teenagers. And recent research on vaping has highlighted risks: a 2015 study found that e-cig vapor damages the immune system of mice. Last year, another study found that e-cigs produce 31 harmful chemicals, including some that are believed to cause cancer. But less is known about what vaping does to the heart and blood vessels.

The scientific community has been divided

The study participants were ages 21 to 45; 16 people who had been vaping for at least a year, and 18 people who didn’t use nicotine at all. The researchers found that e-cig users had an increase in adrenaline levels in the heart that can predispose smokers to bad heart rhythms, heart attacks, and sudden death. They also found that vapers had increased oxidative stress, which can lead to changes in blood fats and lead to arteriosclerosis. Both are known risk factors in cigarette smokers, but it’s not entirely clear what is causing them in electronic cigarettes.

E-cigs have different solvents and flavorings, and most of them contain nicotine. The e-cigs smoked by the study participants all contained nicotine, and Middlekauff and others believe that nicotine is the major culprit for the body changes recorded in the study.

The study has some limitations — it is small, which makes it less reliable. And the participants were reporting their own habits, sometimes dishonestly: a few people were removed from the study because though they claimed they didn’t smoke tobacco cigarettes, carbon monoxide levels in their blood tests said otherwise. And the findings don’t connect the dots all the way to actual heart disease — they only show that vaping affects the markers we use as risk factors, says Gordon Tomaselli, the chief of the cardiology division at Johns Hopkins University and a former American Heart Association president, who did not take part in the study. “These surrogate markers … are cautionary tales,” he tells The Verge, “and I think warrant longer-term follow-ups.”

“We lack a scale.”

Additionally, the study didn’t include a control group made of tobacco cigarette smokers, which could have added interesting information on the health effects of e-cigs vs. regular cigarettes, says Aruni Bhatnagar, a professor of medicine at the University of Louisville, who studies oxidative stress and did not take part in the study. “It would be very nice to know what the magnitude of the injury is,” he tells The Verge. “We lack a scale.” Middlekauff says that she’s already recruiting tobacco smokers for a future study. “It’s a different research question,” she says.

By identifying the cardiovascular risks, today’s study adds a new piece to the puzzle that could be used to inform the public, and especially teenagers and their parents, about the harms linked to e-cigarettes, says Roberto Carnevale of the Sapienza University of Rome, Italy, who has studied the cardiovascular effects of e-cigs. “The theme of e-cig safety is timely and important,” Carnevale wrote in Italian in an email to The Verge. (I’m a native speaker of Italian.) “To this day, in fact, we don’t know the potentially damaging health effects of electronic cigarettes, especially the long-term effects.”