Almost two-thirds of teens and young adults who use Juul e-cigarettes don’t know they contain nicotine, according to a new study.

Juul cartridges, which look like a USB flash drive, have nicotine levels equal to a pack of cigarettes, researchers from the non-profit health organization Truth Initiative report in the journal Tobacco Control.

Juul now commands more than half of the e-cigarette market, HealthDay reports.

“It is no wonder that Juul e-cigarettes have rapidly caught on with youth–they look like a sleek USB flash drive, are easily concealed, and come in youth-appealing flavors like mint, mango and creme brulee,” Robin Koval, CEO and president of Truth Initiative said in a news release. “Unfortunately, young people are unaware that Juul packs a powerful nicotine punch with a single cartridge equal to an entire pack of cigarettes. This escalates the urgency for Food and Drug Administration regulation and public education regarding the risks for young people.”