E-cigarette use among teens tripled from 2013 to 2014, a new government report finds. An estimated 13 percent of high school students used e-cigarettes last year—compared with 9 percent who smoked traditional cigarettes.

The report, from the Centers for Disease Control and Prevention (CDC), found about one-fourth of high school students and 8 percent of middle school students used tobacco in some form in 2014. Between 2011 and 2014, the share of high school students who smoked traditional cigarettes dropped from 16 percent to 9 percent.

An additional 400,000 young people used a tobacco product in 2014, due to a marked increase in the use of e-cigarettes and hookah pipes.

While some teens say they are using e-cigarettes to help them quit using traditional cigarettes, others say they use e-cigarettes because they enjoy the taste and like being part of the trend. Teens interviewed by The New York Times said two of their favorite flavors were “Sweet Tart” and “Unicorn Puke.”

Critics of e-cigarettes say they encourage young people to smoke by introducing nicotine, an addictive substance, to teens. They note e-cigarettes make the act of puffing on a tobacco product seem normal.

“This is a really bad thing,” said CDC Director Dr. Thomas R. Frieden. “This is another generation being hooked by the tobacco industry. It makes me angry.” He noted research has shown that nicotine harms the developing brain.

The 25 percent drop in traditional cigarette use by teens between 2013 and 2014 suggests e-cigarettes may be helping some teens quit, the article notes. “They’re not a gateway in, and they might be accelerating the gateway out,” said David B. Abrams, Executive Director of the Schroeder Institute for Tobacco Research and Policy Studies, an anti-tobacco group.

After the new study was released, public health advocates have been increasing pressure on the U.S. government to quickly regulate and restrict access to e-cigarettes, according to Reuters.