WASHINGTON — E-cigarettes aren’t considered as risky as regular cigarettes, but researchers have found a clue that their flavorings may be bad for the heart.

Longtime smokers who can’t kick the addiction sometimes switch to e-cigarettes, in hopes of avoiding the cancer-causing chemicals in tobacco smoke.

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 vapor 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, said Dr. Joseph Wu, director of Stanford University’s cardiovascular institute.

So his team tried the next best thing for a study published Monday: In laboratory dishes, they grew cells that normally line healthy human blood vessels. They exposed the cells to six different e-cigarette flavorings, testing if the flavors — 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, the way chemicals from vaping would make their way to the cardiovascular system. They also compared the cells’ exposure to blood from nonsmokers and people who smoked a regular cigarette.

Vaping and some flavorings, 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. But overall, cells showed signs of damage and were inflamed, less able to form new blood vessels, or heal wounds.

Small laboratory studies like this one can’t prove vaping really does harm, cautioned Dr. Jane Freedman of the University of Massachusetts, who wasn’t involved in the research. But she said the work should spark additional safety testing.