E-cigarettes could be as bad for you as smoking, according to a new study.
The research, carried out at the University of California, found people who vape were more likely to develop heart problems than non-smokers.
The findings, published in JAMA Cardiology, showed e-cigarette users were at higher risk of having two factors that led to heart disease, increased heart adrenaline levels and more oxidative stress.
Experts compared 23 vapers, aged between 21 and 45, with 19 non-smokers.
The average participant's age was 27.
Dr Holly Middlekauf, from University of California, Los Angeles, said: "Nicotine, which is the major bioactive ingredient in e-cigarette aerosol, with its metabolities, may harbour unrecognised, sustained adverse physiologic effects that lead to an increased cardiovascular risk profile in habitual e-cigarette users."
Professor of Behavioural Medicine at Oxford Paul Aveyard was sceptical about the study.
He said: "Whenever you test several measures, as they did in this study, the greater the chance you find differences that are not real, they are simply down to chance.
"It’s possible to correct for this kind of problem, but the authors did not do this.
In one of the tests, e-cigarette users actually had higher oxidative stress than smokers.
Dr Aveyard considered this could be down to "psychological factors" as some smokers show reduced heart rate after smoking.
He said: "Most people who were using e-cigarettes had smoked in the past, while the large majority of the non-users of e-cigarettes had never smoked.
"People with some psychological disorders or who are stressed show reduced heart rate variability.
"Overall, then, this is a small study with several possible explanations for the findings, only one of which could be that using e-cigarettes is a possible risk for heart disease."
His final advice was: "If a person cannot stop smoking in other ways, public health advice is to switch to e-cigarettes either partially or wholly.
"This small study does not change that advice. Compared with smoking, e-cigarettes are a better option."