After California legalized recreational marijuana use for adults in 2016, teens’ use of the substance also increased, according to a new study.

Researchers evaluated survey data from more than 3 million teens, who answered questions about their marijuana use in the years before and after marijuana was legalized in California. Between the 2017-18 and 2018-19 school years, after marijuana was legalized, there was a 23% increase in marijuana use over the past 30 days. Teens’ likelihood of lifetime marijuana use increased 18%, HealthDay reports.

“I was somewhat surprised to see relatively greater increases in the prevalence of marijuana use among younger adolescents [seventh graders] relative to ninth and 11th graders, among females versus males, among non-Hispanic versus Hispanic youth, and among whites versus youth in other racial groups,” lead researcher Mallie Paschall said in a news release. “In other words, there were greater increases in marijuana use prevalence after recreational marijuana legalization among youth in ‘low-risk’ groups, which is concerning.”