Short-term mediating factors of a school-based intervention to prevent youth substance use in Europe.

To investigate factors mediating the effects of a European school-based intervention (Unplugged) based on a social influence approach to youths' substance use.Schools in seven European countries (n = 143, including 7,079 pupils) were randomly assigned to an experimental condition (Unplugged curriculum) or a control condition (usual health education). Data were collected before (pretest) and 3 months after the end of the program (posttest). Multilevel multiple mediation models were applied to the study of effect mediation separately for tobacco, alcohol, and cannabis use. Analyses were conducted on the whole sample, and separately on baseline users and nonusers of each substance.Compared with the control group, participants in the program endorsed less positive attitudes toward drugs; positive beliefs about cigarettes, alcohol, and cannabis; and the normative perception of peers using tobacco and cannabis. They also increased in knowledge about all substances and refusal skills toward tobacco. Decreased positive attitudes toward drugs, increase in refusal skills, and reappraisal of norms about peer using tobacco and cannabis appeared to mediate the effects of the program on the use of substances. However, mediating effects were generally weak and some of them were only marginally significant.This study lends some support to the notion that school-based programs based on a social influence model may prevent juvenile substance use through the modification of attitudes, refusal skills, and normative perceptions.