Background Estimations of drug use are mostly based on population surveys that can suffer from response biases. The current study evaluates using wastewater-based epidemiology (WBE) for assessing illicit drug use by comparing wastewater data with that from a population survey. Methods Introductory letters (29,083) were sent to inhabitants of Lier, Belgium, asking them to participate in an online survey study. Participants were asked to indicate their drug use in the past week for a 12-week period (September–November 2014). Concomitant wastewater samples were collected from the associated wastewater treatment plant in four bi-weekly periods. Samples were analyzed using solid-phase extraction and liquid chromatography coupled to tandem mass spectrometry (LC–MS/MS). Results On average, 263 (1%) inhabitants filled out the questionnaire each week. According to the survey results, cannabis was the most used drug, followed by amphetamine, cocaine and methylenedioxymethamphetamine (MDMA). Wastewater data corroborated these results. Cocaine, amphetamine and MDMA showed a significant difference between days of the week. The four sampling periods differed significantly from each other for cocaine, amphetamine and methadone. Conclusion Observed drug consumption patterns from survey and wastewater data match national and international data. Wastewater analyses confirm that WBE can be reliably used to confirm patterns and trends in drug use. Future studies should focus on identifying the most opportune sampling period giving the most reliable estimates of drug use and use smaller, contained communities such as festivals or prisons if methodology allows.