"Sin taxes" have been extensively used to correct consumers' behaviors, in an attempt to make them internalize any externality or internality caused by the consumption of harmful goods. However, the debate on the effectiveness of these measures is still ongoing when specific products are concerned. In particular, sugar taxes have exhibited mixed evidences in terms of effects on purchasing behaviours and health outcomes. The aim of this project is to analyse whether the health status of Finnish childrens and adolescents was influenced by the Excise Duty on Soft Drinks (Law n.1474/1994) in terms of body mass index, overweight and obesity prevalence and other measures of healthy growth. We exploit the introduction of this duty as a natural experiment in order to build a difference-in-difference framework. We use a Finnish 1966 cohort as a baseline and compare it to a Finnish 1986 (treated) cohort, which experienced the introduction of the duty when children were aged 9. We find that the duty had a negative effect on the healthy growth of 1986 childrens (higher body mass index, higher prevalence of overweight, lower height). We provide possible mechanisms, of behavioural and socioeconomic nature, to explain these unexpected effects on youth's health.