Innovation is increasingly important in the delivery of efficient healthcare; however, the pathways through which medical innovation reach clinical practice are fraught with uncertainty. Further scientific investigation and technological development are normal in medical innovation even when drugs, diagnostics or medical procedures are already adopted. Diffusion studies rarely admit such possibilities as a fundamental element of their conceptual frameworks. This paper explores the diffusion process of an antiretroviral drug (Abacavir) and the introduction of its companion pharmacogenetic test into clinical practice in Italy. This is a landmark case of translational medicine where the principles of pharmacogenetics made an important - applied - contribution to medical innovation shifting the focus away from the diffusion of a complete technology (the drug) towards that of a dynamic technology (Abacavir + developing companion diagnostics). We adopt the historical method to analyse the sequence of events. Key findings show that the diffusion process of a dynamic technology does not fit in the widely accepted S-shaped model for a complete technology. The diffusion phase presents complex interactions amongst the stakeholders involved, each operating on the basis of their own competences within the environment and the regulatory system; the analysis of the diffusion process should proceed through the correct identification of the dynamic technology – the therapy - and cannot be de-coupled from scientific discovery and technological development.