In summary, this study revealed the widespread occurrence of antiviral drugs in the aquatic environment. Furthermore, it could be shown that the elimination of pharmaceuticals in both biological and oxidative treatment do not necessarily result in their mineralization but rather leads to the formation of a variety of transformation and oxidation products. This is one of the first studies in which the fate and in particular the transformation of pharmaceuticals has been comprehensively investigated in almost the complete water cycle, from biological wastewater treatment to advanced oxidation processes via ozone. It was shown that the transformation of pharmaceuticals in the urban water cycle can ultimately result in the formation of toxic transformation products.