Wastewater analysis has been taken up with enthusiasm in the illicit drugs field. Through a critical social science lens, we consider claims to what these ‘promising’ methods might afford in the context of drug epidemiology and policy, recognising that all methods have social effects in their specific contexts of use. We outline several ethico-political issues, highlighting how methods can have different effects as they move from one discipline (environmental science or analytic chemistry) into another (illicit drugs). Translated into the drugs field, wastewater analysis problematically shifts the focus of drug policy from harm reduction to drug use prevalence and entrenches stigma. Without comprehensive information about the social and contextual aspects of drug harms, effective drug policy is not possible.