Illicit drugs and metabolites have been recognized recently as a group of emerging contaminants of concern, as a consequence of their high volumes of use and production. Drug residue levels in the aquatic environment have also been pointed out as good indicators of illicit drug abuse. The present work assesses for the first time drug abuse in a penal complex from the levels of different drug residues measured in the prison sewage waters and evaluates the suitability of this approach to track and control illicit drug usage in such facilities. The presence of various drugs of abuse and metabolites in sewage waters from a penal complex was determined by an analytical method based on on-line solid phase extraction-liquid chromatography-tandem mass spectrometry. Levels of consumption indicators measured in this water were used to back-calculate drugs usage in the penal complex. Daily use was observed for methadone (average of 156 doses/day/1000 inh), alprazolam (129 doses/day/1000 inh), ephedrine (46 doses/day/1000 inh), cannabis (33 doses/day/1000 inh.) and cocaine (3 doses/day/1000 inh). Sporadic consumption was observed for heroin, amphetamine, methamphetamine, and ecstasy. In spite of the fact that this and other sewage epidemiological approaches described may suffer from bias that still need to be investigated and refined, it provides near "real-time" information on collective drug use in an anonymous way and constitutes a very useful, economic and fast tool to evaluate the efficiency of measures adopted to control and track drug abuse in this type of facilities (or any other provided that has a STP associated or an accessible collector system).