Wastewater-based epidemiology is a well-established and complementary approach for monitoring illicit drug use in the general population. In this study, amphetamine (AMP), methamphetamine (METH), 3,4-methylenedioxymethamphetamine (MDMA), cocaine (COC) (from benzoylecgonine), and cannabis (from THC-COOH) consumption levels were investigated for the first time in Turkey (Istanbul). A solid-phase extraction method was applied to influent wastewater samples collected from two districts, Beyoglu and Catalca. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was conducted with electrospray ionization in positive mode. Calibration curves were acquired in linear form with >0.999 correlation coefficients. Limit of detection levels were measured as 0.91-151 ng/L, and limit of quantitation levels were in the range of 3 to 500 ng/L. Solid-phase extraction recovery and repeatability experiments were achieved by spiking the mix solution to different concentrations (50, 250, 750 ng/L) in 50 mL tap water and wastewater (500, 1000 ng/L) samples in six replicates. The method was optimized, and recoveries were found to be over 80% for all six substances with up to 11.9% relative standard deviation. According to the real sample results, cannabis was found to be the most abused illicit substance among the analytes. The mean consumptions of the two districts, including seven consecutive days for AMP, METH, MDMA, COC, and cannabis, were found to be 27.2, 322, 331, 385, and 1224 mg/day/1000 inhabitants, respectively. In this presented study, all targeted compounds were analyzed simultaneously with the same analytical conditions. To the best of our knowledge, this report is the first to present illicit drug consumption data from Istanbul. (C) 2018 Elsevier B.V. All rights reserved.