The stability of five illicit drug markers in wastewater was tested under different sewer conditions using laboratory-scale sewer reactors. Wastewater was spiked with deuterium labelled isotopes of cocaine, benzoyl ecgonine, methamphetamine, MDMA and 6-acetyl morphine to avoid interference from the native isotopes already present in the wastewater matrix. The sewer reactors were operated at 20°C and pH 7.5, and wastewater was sampled at 0, 0.25, 0.5, 1, 2, 3, 6, 9 and 12h to measure the transformation/degradation of these marker compounds. The results showed that while methamphetamine, MDMA and benzoyl ecgonine were stable in the sewer reactors, cocaine and 6-acetyl morphine degraded quickly. Their degradation rates are significantly higher than the values reportedly measured in wastewater alone (without biofilms). All the degradation processes followed first order kinetics. Benzoyl ecgonine and morphine were also formed from the degradation of cocaine and 6-acetyl morphine, respectively, with stable formation rates throughout the test. These findings suggest that, in sewage epidemiology, it is essential to have relevant information of the sewer system (i.e. type of sewer, hydraulic retention time) in order to accurately back-estimate the consumption of illicit drugs. More research is required to look into detailed sewer conditions (e.g. temperature, pH and ratio of biofilm area to wastewater volume among others) to identify their effects on the fate of illicit drug markers in sewer systems. © 2013 Elsevier Ltd.