Viewing ads for medical marijuana may influence middle school students to use the drug, a new study suggests. Students in sixth through eighth grades who saw ads for medical marijuana were twice as likely as others to have used the drug or to say they intended to use it in the future, the study found.

The study included more than 8,200 middle school students in Southern California, HealthDay reports. In the first year of the study, 22 percent of the students said they had seen at least one medical marijuana ad in the past three months. The following year, the number rose to 30 percent.

The findings are published in Psychology of Addictive Behaviors.

“As prohibitions on marijuana ease and sales of marijuana become more visible, it’s important to think about how we need to change the way we talk to young people about the risks posed by the drug,” study author Elizabeth D’Amico of the nonprofit research organization RAND said in a news release. “The lessons we have learned from alcohol — a substance that is legal, but not necessarily safe — may provide guidance about approaches we need to take toward marijuana.”

Teens can see ads for medical marijuana on billboards, in newspapers and on television, and on dispensary storefronts, the researchers noted.

“Given that advertising typically tells only one side of the story, prevention efforts must begin to better educate youth about how medical marijuana is used, while also emphasizing the negative effects that marijuana can have on the brain and performance,” D’Amico said.

The researchers said their findings do not prove that seeing ads cause marijuana use. “However, the study does raise questions about whether there is a need to revise prevention programming for youth as the availability, visibility and legalization surrounding marijuana changes,” they noted.