Effectiveness of a cannabis prevention program in school: results of a randomized prospective study

Background: In the current debate about legalized access to cannabis for adults in Germany, there is widespread agreement about the need for increased prevention among children and young people. The aim of this work is to examine the effectiveness of a school-based cannabis prevention program on knowledge, attitudes, and behavior. Method: Cluster-randomized waiting list control group study with two study arms: “participation in the ‘Der grüne Koffer’ prevention program”, a collection of methods for cannabis prevention, vs. “no participation” and two measurement time points before and four months after the intervention in four German states. Results: The mean age of the sample of 3976 adolescents from 280 school classes in grades 8 and 9 was 14.02 years (standard deviation = 0.89). The sex ratio was balanced (49.5% female). Cannabis-related knowledge increased significantly more in the intervention group compared to the control group from the first to the second measurement time point (adjusted beta = 0.25 [95% confidence interval: 0.14–0.37]). Attitudes toward cannabis use did not change significantly in either group. In the 8th grades of the intervention group, significantly fewer adolescents (2.9%) started experimenting with cannabis use for the first time during the observation period than in the control group (5.3%), whereas no group difference was detectable in the 9th grades (interaction grade level x group: odds ratio = 2.17 [1.13–4.15], p = 0.019). Discussion: The evidence of an effect of participation in the “Der grüne Koffer” prevention program on knowledge and first-time use of cannabis in the 8th grade can be considered promising against the background of previous national and international studies, which could not provide evidence of effects of school-based cannabis prevention in middle and late adolescence.