This study proposes an approach integrating current methodology, which determines the amount of drug consumed per month/day per 1,000 inhabitants by measuring concentrations of illicit drugs and their metabolites in untreated wastewater, with geographic information system (GIS) designed for this specific purpose. The proposed approach contributes to existing studies by the use of multi-criteria decision-making (MCDM) technique which enables the user to make spatial decisions based on analyzing the effecting parameters of illicit drug use. Usability of GIS technology for monitoring illicit drug consumption was showed in this study by applying the proposed methodology in a pilot study area. MCDM of the model established in the case study included four parameters: building use-cases, age, gender and population living in the study area. Weights of the stated parameters have been estimated using the analytic hierarchy process technique. As a result of the study, the proposed methodology produced a thematic map that classifies the monitoring area according to the potential for illicit drug use. Initial results of the proposed system produced by using sample data of a district in Istanbul were presented in this paper. Also, the challenges that may be encountered in the comprehensive applications of the proposed system was discussed in this study.