Teens who use Juul brand e-cigarettes often don’t realize their addictive potential, according to a new study by Stanford University researchers.

The study of 10 California high schools found teens who used Juul e-cigarettes reported vaping a significantly greater number of days compared with teens who smoked regular cigarettes.

“I was surprised and concerned that so many youths were using Juul more frequently than other products,” study senior author Bonnie Halpern-Felsher said in a news release. “We need to help them understand the risks of addiction. This is not a combustible cigarette, but it still contains an enormous amount of nicotine — at least as much as a pack of cigarettes.”

Juul accounts for two-thirds of the U.S. e-cigarette market, HealthDay reports.