Aims – Finnish treatment of drug abuse has during the last two decades shifted from a predominantly psychosocial approach to a more medical mode. This is especially evident in the rapid implementation of substitution treatments (STs). My aim is to show that labelling this development as ‘medicalisation’ or ‘rationalisation’ as a form of medical progress will not increase our understanding of the change. Material and design – I analysed texts from several periodicals with psychosocial, social policy and medical perspectives between 1997 and 2005. Four basic conceptual and argumentative underpinnings emerged which gave credence to the medical and rational approach, and the validity of these four elements was then investigated. I also collected all the texts on drug addiction and its treatment from two medical journals in 1965–1976 to examine the way in which drug addiction was conceptualised during this earlier phase. Results – The material shows that there are at least four reasons why medicalisation and rationalisation cannot explain the implementation of substitution treatments in Finland. First, progress in medical research on addiction did not make STs necessary. Second, the effectiveness of substitution treatments hinges on a particular kind of scientific rationality that cannot be equated with rationality per se. Third, it was not the 1990s and 2000s that drug addiction was coded as a medical problem for the first time. Fourth, it is difficult to include into the medicalisation theory how people actively want to be ‘medicalised’. Medical knowledge and technology open up new domains of knowledge with possible relations to practices of power and offer people new ways of self-understanding. How these different practices work is a question of empirical research. Both ‘rationalisation’ and ‘medicalisation’ are concepts often used in an inflationary way, and this may make them insensitive as analytical instruments. Keywords – Drug addiction, history, medicalisation, rationalisation, social theory, substitution treatment.