The risk-benefit balance of a drug is never finalized, and new risks may emerge after the drug has been approved for use. Making such emergent risks matter in patient care is difficult, however. In the process of transforming a pharmacoepidemiological signal to practical knowledge in patient care, information about an emergent risk travels through different regulatory, scientific and clinical contexts and cuts across the multiple ontologies of a medical product. Based on empirical studies of how drug manufacturers and regulators cooperate to produce warning letters to physicians and how physicians read and react to such a letter, this talk traces a genre of risk rhetoric to describe the transformation that emergent risk knowledge undergoes and the networks that assemble around it.