SEATTLE— Goodbye cotton candy, fruit medley, and crème brûlée. The tantalizing flavors that may have helped to drive the surge of e-cigarette use among teens are on their way out in the United States. Companies that make vaping liquids must now comply with a U.S. Food and Drug Administration policy released last month banning vape liquids that taste like anything but tobacco or menthol.

But vaping still delivers something highly appealing and addictive: nicotine. And researchers are just beginning to study the drug’s long-term impact on the developing brain.

On Friday, at the annual meeting of AAAS, which publishes Science , Marina Picciotto, a neuroscientist at Yale University, described some of her early findings, mostly in animals. Because vaping gained popularity mostly within the past decade, “We don’t know yet what the consequences are going to be for the long-term intake of vape fluids or the nicotine in vaping,” she said. But clues are emerging.

One thing is well-known from adult smokers: nicotine is highly addictive. Some cartridges of vape liquid can contain as much nicotine as a pack of cigarettes. Nicotine “takes things you kind of like and it makes you like them more,” Picciotto said. For teens, vaping may be associated with a flavor, and these associations make nicotine more pleasurable and more desirable. “Nicotine enhances the rewarding effects of flavors or other stimuli that are slightly rewarding,” she said. For teens, those can be a piece of music or a sexual experience—cementing the link between nicotine and pleasure.

So far, Picciotto has researched the long-term effects of nicotine exposure on developing brains in mice. This research showed nicotine-exposed adolescent mice had structural changes to their brain cells, altering how information is sent throughout the brain. These nicotine-exposed mice were more sensitive to stress and responded to very low stimuli that didn’t faze other mice. For example, mice who received a weak electrical shock to their foot reacted, whereas unexposed mice didn’t even acknowledge they had been shocked. In a human study, Picciotto observed similar behaviors, where children who were exposed to nicotine before birth were more likely to have an emotional overreaction to stress.

Picciotto said these findings suggest that developing adolescents who are exposed to nicotine may have structural changes to their brain that negatively impacts behavior later in life. Still, she said, more research is needed to understand the complex relationship between behavioral problems in adulthood and nicotine exposure.

But nicotine isn’t the only harmful ingredient in vape liquids. Most of the 64 vape-related deaths and almost 3000 vape-related hospitalizations recorded by the U.S. Centers for Disease Control and Prevention are likely because of custom-made and illegally distributed vape liquids that use a form of vitamin E oil. Vitamin E oil is made up of fats, and Picciotto said fats are known to inflame the lungs. This would explain the lung illnesses that are hospitalizing people who vape. But even commercial liquids contain ingredients such as polyethylene glycol that Picciotto said don’t belong in the lungs. “The issue of what is in vape liquid is one that’s very important,” she said.

Picciotto thinks vaping companies should be required to include a full list of ingredients—and the percentages of those ingredients—on all packaging. “We have labeling of all the food that we get in the supermarket,” Picciotto said. “Why would we not demand that we have labeling of every constituent in the vape liquid?”

She also favors public health campaigns that would clearly identify the risks of vaping, including how nicotine addiction impacts adolescents. Picciotto acknowledges that for adult smokers, switching to vaping can be beneficial. But no one who takes up vaping, especially not teens, should believe that “they are inhaling harmless steam.”