An increasing number of teens in the United States are vaping marijuana, according to a new study.

Between 2017 and 2019, the percentage of teens who said they used marijuana in the past month rose from 13.9% to 15.4%. Those who said they vaped marijuana at least once a week more than doubled, from 2.1% to 5.4%. The proportion of teens who said they smoke marijuana declined, HealthDay reports.

Past research points to why teens are switching from smoking to vaping marijuana, said Linda Richter, vice president of prevention research and analysis for Partnership to End Addiction, who was not involved in the study. “First, the wave of nicotine vaping that spread throughout middle and high schools over the past several years increased the likelihood that kids who vape nicotine will transition to vaping marijuana,” she said.

“Vaped products are also typically seen as safer and healthier than smoked products, because when e-cigarettes were originally introduced to the market, they were advertised as safer alternatives to cigarette smoking,” Richter added.

However, she said, vaping marijuana can deliver a higher dose of THC than smoking marijuana. The higher THC dose can “hook young people to the substance and keep them coming back for more,” she said.