Preventing tobacco use from the start: Short- and medium-term impacts on the youth

•We evaluate a substance-use prevention program implemented in Iceland (1997–2002).•We analyse short- and medium-term impacts on smoking prevalence by gender.•To evaluate the policy, we construct a ‘synthetic control’ for Iceland.•Smoking prevalence fell following the policy, more so among girls.•We find positive externalities in age groups adjacent to those directly treated.