This paper offers the first case study of Phospho-Energon — an early twentieth-century Swedish patent medicine believed to cure nervousness. Using a large dataset of newspaper advertisements, it explores how the product was presented through scientific and medical language, which drew upon a range of visual and verbal rhetoric to convince consumers of its benefits. It finds that pseudoscientific discourse focusing on self-help was regularly used to sell Phospho-Energon, with consumers warned that their nerves were “not allowed to fail” and required “protection” in order to remain healthy. Furthermore, the “science” supporting this discourse gradually shifted over time as neurosis replaced neurasthenia as a diagnostic category and the concept of spring lethargy became popularised. Overall, this study argues that Phospho-Energon stands as an important example of how partial scientific/medical claims can be used as a rhetorical device to sell products to consumers looking for a quick-fix cure for their perceived mental health conditions.