E-cigarettes aren't considered as risky as regular cigarettes but researchers have found a clue indicating their flavourings may be bad for the heart.
Longtime smokers who can't kick the addiction sometimes switch to e-cigarettes in the hope of avoiding the cancer-causing chemicals in tobacco.
But cigarette smoking doesn't just cause lung cancer. It's a leading cause of heart attacks too, and little is known about e-cigarettes and heart disease.
Chemicals in the inhaled vapour may pose unique risks that are important to understand, especially as more and more teens take up vaping.
"It's not possible for me to go into a patient and strip their artery and test it" for a reaction to vaping, Dr Joseph Wu, director of Stanford University's cardiovascular institute, said.
In laboratory dishes, Wu's team grew cells that normally line human blood vessels and exposed the cells to six different e-cigarette flavourings, testing if the flavours - and not just the nicotine - caused any effects.
They also tracked what happened when those cells were bathed in blood taken from people right after they had an e-cigarette.
Vaping and some flavourings, even without nicotine, triggered blood vessel dysfunction that can increase the risk of heart disease, the researchers reported in the Journal of the American College of Cardiology.
Cinnamon and menthol seemed the most toxic.
The findings "suggest that even without the smoke of combustible cigarette products, there may be a smouldering fire of adverse health effects," Dr Jane Freedman of the University of Massachusetts wrote in an accompanying editorial.
Another study at a recent heart meeting looked at health records to conclude e-cigarette users had a higher risk of heart attack than people who neither vape nor use tobacco products, but that too was only a clue.
Wu's team plans additional studies.
US public health officials are alarmed by an explosion of underage vaping, but Wu said it's not just a question for teens.
He worries about people who already have heart disease and may think switching from tobacco to e-cigarettes is enough protection.
"This is really a warning shot that people should not be complacent and think that these e-cigarettes are completely safe," Wu said.
Australian Associated Press
