Do school-based smoking preventive interventions have unintended effects? Post hoc analysis of the Focus cluster randomised controlled trial

Abstract Objectives: Public health interventions are designed to improve specific health-related outcomes; however, they may also produce negative side effects, such as substitution use, psychological or social harms. Knowledge about the unintended effects of school-based smoking preventive interventions is sparse. Hence, this study examined these potential unintended effects of the smoking-reducing intervention, Focus, among students in the vocational education and training setting. Design: Cluster randomised controlled trial stratified by school type with 5 months follow-up. Setting and participants: Across Denmark,