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BACKGROUND: As prevalence of tobacco use falls, socioeconomic inequalities in tobacco use are increasing in many high-income countries. Evidence is lacking on the effect of preventive interventions on socioeconomic inequalities in smoking initiation among adolescents. We evaluated whether a multicomponent school-based prevention programme with parental involvement has differential effects on smoking initiation across socioeconomic groups and affects the magnitude of socioeconomic inequalities in smoking initiation.

METHODS: A secondary analysis of data from a 3-year cluster randomised controlled trial, the TOPAS study, conducted in Sweden from 2018 to 2021. Schools were randomised either to the full programme (Tobacco-Free Duo, T-DUO) or minimal intervention (EDU). The analysis was conducted according to intention to treat for the primary outcome, the probability of remaining a non-user of cigarettes at the end of compulsory school (ages 15-16). Parents' educational attainment was the socioeconomic variable. Differential effects were analysed by comparing adolescents exposed to T-DUO with those exposed to EDU within each socioeconomic group. The effect of the intervention on the magnitude of inequalities was analysed by comparing several measures of absolute and relative inequalities between T-DUO and EDU.

RESULTS: At the end of follow-up, the full programme had a similar, at most moderate effect on smoking initiation in all socioeconomic groups (relative risk 1.13 (95% CI 1.02 to 1.25) in the middle group). The programme did not significantly affect the magnitude of inequalities (Slope Index of Inequality difference 1.49 (95% CI -15.34 to 18.32)).

DISCUSSION: Socioeconomic inequalities in smoking initiation remain substantial. Our results indicate the absence of an effect of the programme T-DUO on these inequalities.