The stability of nine illicit drugs and metabolites in influent wastewater at relevant conditions (20 C and at pH 7.5) was evaluated during 26h (in 1 h intervals) to assess whether significant in-sewer processes of degradation or formation occur. This assessment is important for the sewage epidemiology approach, which uses concentrations of illicit drugs and metabolites in influent wastewater to back-calculate amounts of these substances used by a community or population. The results of this study showed that amphetamine, methamphetamine, ecstasy and EDDP (metabolite of methadone) are stable in wastewater for 12h and longer. For benzoylecgonine and methadone, a modest formation in the course of time was observed (0.2% per h and 0.4% per h, respectively), while for cocaine (40% degradation after 12 h), ecgonine methylester (20% degradation after 12 h), and 6-monoacetylmorphine (20% degradation after 12h), a clear decrease in concentrations in time was seen. These findings suggest that for compounds with a significant stability issue, it is important to take their behavior into account when performing back-calculations in sewage epidemiology. However, this study also highlights the need of future research regarding transformation pathways of the individual compounds. (C) 2012 Elsevier B.V. All rights reserved.