Friends' and parents' tobacco use are strong predictors of tobacco uptake among adolescents, however the effectiveness of interventions based on public commitments and agreements to remain tobacco-free are not established. Here, we evaluated the effectiveness of the school-based Swedish program Tobacco-Free Duo (T-Duo) in preventing adolescents from initiating tobacco use (TOPAS study). T-Duo is a multi-component intervention with is a formal agreement between a student and an adult partner to remain tobacco-free during the entire 3-year study period as core component. The standardized educational component of the same program was used as comparator (control). Primary outcome was the probability to "remain a non-user" of i) cigarettes and secondary outcomes ii) other types of tobacco at second (21-month) follow-up. Analysis was conducted according to Intention To Treat. In total 1776 adolescents (51% female) aged 12-13 in grade 7 from 34 participating high schools in Sweden were included at baseline in 2018, of which 1489 were retained after 21 months. The Risk Ratio (RR) of not having tried cigarettes 21-months after initiation of the intervention was 1.03(CI 0.98-1.08), Bayes Factor(BF) = 0.93, Absolute Risk Difference(ARD) = 3.1%. Similar associations were found for never smoked a whole cigarette and never use of other tobacco/nicotine products. There was a minimal reduction of tobacco use initiation among Swedish adolescents assigned to a multi-component intervention (T-Duo) compared to those assigned to standardized classroom education after 2 schoolyears. However, for most outcomes' findings were inconclusive and not reliably different from zero.