Is universal prevention against youths' substance misuse really universal? Gender-specific effects in the EU-Dap school-based prevention trial

Background: Studies of effectiveness of school-based prevention of substance misuse have generally overlooked gender differences. The purpose of this work was to analyse gender differences in the effectiveness of a new European school-based curriculum for prevention of substance misuse among adolescents. Methods: The European Drug Abuse Prevention (EU-Dap) trial took place in seven European countries during the school year 2004-05. Schools were randomly assigned to either a control group or a 12-session standardised curriculum ("Unplugged") based on a comprehensive social influence model. The analytical sample consisted of 6359 students (3324 boys and 3035 girls). The use of cigarettes, alcohol and illicit drugs, adolescents' knowledge and opinions about substances, as well as social and personal skills were investigated through a self-completed anonymous questionnaire administered at enrolment and 3 months after the end of the programme. Adjusted Prevalence Odds Ratios were calculated as the measure of association between the intervention and behavioural outcomes using multilevel regression modelling. Results: At enrolment, boys were more likely than girls to have used cannabis and illicit drugs, whereas girls had a higher prevalence of cigarette smoking. At the follow-up survey, a significant association between the programme and a lower prevalence of all behavioural outcomes was found among boys, but not among girls. Age and self-esteem emerged as possible modifiers of these gender differences, but effects were not statistically significant. Conclusions: Comprehensive social influence school curricula against substance misuse in adolescence may perform differently among girls and boys, owing to developmental and personality factors.