The vape epidemic is still blowing up.

There were 1.5 million more middle school and high school students using e-cigarettes in 2018 than in 2017, research from the University of North Carolina at Chapel Hill School of Medicine published on Friday showed, as the teen vaping trend continues. Young people’s attraction to vapes — which have been marketed as an alternative to traditional smoking for adults — has to do largely with the fruity flavors they come in, the study found.

“The review shows that e-cigarette flavors appear to motivate e-cigarette and smoking initiation among youth,” said Adam Goldstein, director of UNC Tobacco Intervention Programs and an author of the study. “The use of flavored e-cigarettes present a dangerous gateway to nicotine addiction for youth, and we know nicotine is particularly harmful to adolescent brain development.”

Goldstein said that based on this study, which surveyed 51 studies on e-cigarettes and flavors in a systematic review, he suggests a ban e-cigarette flavors. FDA commissioner Scott Gottlieb has been adamant about addressing the vaping epidemic, which he called “disturbing” and particularly worrying as a survivor of cancer himself. The long-term health effects of vaping are not yet clear, though studies show it can lead teens to using cigarettes.

“The thought of any child starting down a path of a lifelong addiction to tobacco, which could ultimately lead to their death, is unacceptable,” Gottlieb said in March 2018. “We need to take every effort to prevent kids from getting hooked on nicotine.”

JUUL, an e-cigarette company valued at $15 billion, has taken a number of measures to stop teens from using its products after the U.S. Food and Drug Administration considered banning flavors to reduce youth e-cigarette use. In November 2018, it shut down all social media to avoid attracting young people and began to pull flavored pods from stores across the country. Nicotine liquid flavors like cucumber and mango are now only available on its website with age verification while classic tobacco, mint, and menthol flavors will still be sold in stores.

Another recent study published in the Journal of the American Medical Association found that teens who had vaped were more likely to try traditional cigarettes.

JUUL spokesman Ted Kwong told MarketWatch that the company launched the flavored pods to help adult smokers to switch from cigarettes. It stopped selling flavored pods in retail stores starting in November 2018 to “combat underage use.” These flavors can be purchased on JUUL’s website now using an age verification technology. The company also supports raising the minimum purchase age for cigarettes, tobacco and vaping products to 21, Kwong said.

Gottlieb also said the FDA may eventually ban menthol cigarettes in addition to flavored vape pods. Studies show more than half (54%) of youth smokers ages 12-17 use menthol cigarettes, compared to less than one-third of smokers ages 35 and older, according to the FDA.

“If youth trends don’t move in the right direction, we will revisit all of these issues,” he said.