Two brief talks with an adult about marijuana can reduce use of the drug by up to 20 percent in teens who are regular users, a new study suggests.

Researchers at the University of Washington studied a two-session approach called Teen Marijuana Check-Up, designed to encourage teens to reduce their marijuana use. The researchers gave short presentations in high school classrooms about the myths and facts of marijuana. The program covered why teens smoke marijuana, and what the health and behavior consequences are.

Science Daily reports that students who heard the presentations were given the opportunity to volunteer to have two one-on-one meetings with a health educator. The researchers chose 310 students from six Seattle high schools to participate. All were regular marijuana users. Each meeting lasted 30 to 60 minutes over two weeks. Some students were counseled using motivational interviewing, in which the student and health educator talked about the student’s marijuana use and how it might be affecting the student’s life. A second group used a PowerPoint presentation about marijuana.

The group who were counseled using motivational interviewing decreased their marijuana use by 20 percent three months after the sessions; after one year, they still smoked 15 percent less compared with the beginning of the study. Those in the PowerPoint group reported an 8 percent decrease after three months, and an 11 percent decrease after a year, the researchers reported in the journal Psychology of Addictive Behaviors.