Over 8000 pharmaceuticals, with more than 3000 active substances, are currently authorized in Switzerland. Transformation products – arising from biological and chemical degradation processes – can sometimes be more toxic to aquatic organisms than the parent compounds; they may also be problematic as a result of their higher mobility. In a case study, scientists from Eawag and the Eawag-EPFL Ecotox Centre have shown that more data is urgently required to assess the environmental risks of transformation products. While structure-based models can be used to predict the toxicity of transformation products from that of the parent compound, larger datasets are needed to validate these models. The authors conclude that data on the ecotoxicological effects of transformation products should be systematically collected in authorization procedures for new chemical products. Only if such data is made available can the various assessment methods be adequately compared, providing a basis for appropriate recommendations.