The findings of the largest European project to date in the emerging science of wastewater analysis are taken up in this ‘Perspective on drugs’. The project in question analysed wastewater around 70 European cities and towns (hereinafter referred to as ‘cities’) to explore the drug-taking habits of those who live in them. The characteristics of European cities differ considerably; with some being major tourist or business hubs with varying day and night time populations. The results provide a valuable snapshot of the drug flow through the cities involved, revealing marked geographical variations.