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Wastewater-based epidemiology (WBE) employs the analysis of human metabolic biomarkers in influent wastewater (IWW) to estimate community-wide exposure to xenobiotics (e.g. prescription opioids). The low ng/L range of concentrations of these biomarkers and the complex matrix composition pose bioanalytical challenges related to sample preparation and detection/quantification. Therefore, a sensitive analytical method for the detection and analysis of 19 opioid biomarkers was optimized and validated according to the European Medicines Agency guidelines. Oasis HLB cartridges were used for sample concentration and an Atlantis T3 column with gradient elution resulted in sufficient separation of the analytes. Absolute recoveries (RE) were highly reproducible and ranged between 50 and 93% with the exception of 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP). The lower limit of quantification (LLOQ) ranged between 1 and 100 ng/L and was based on the analyte concentrations found in IWW. Process efficiency was acceptable for all biomarkers for which an isotope-labelled deuterated analogue was available. All biomarkers showed high benchtop stability with the exception of buprenorphine, EDDP, fentanyl and normorphine. Apart from buprenorphine and hydrocodone, all analytes under investigation were detected at least once above LLOQ levels in five locations in Belgium. The highest population-normalized mass loads were found for tramadol, O-desmethyltramadol and codeine. The proposed methodology was able to evaluate spatial differences in opioid use. [Display omitted] • Validation of an analytical method for quantification of 19 opioids in wastewater. • Process efficiency acceptable for all analytes when deuterated analogue was available. • High in-sample stability for 15 opioid biomarkers. • Spatial differences in opioid use between five Belgian catchment areas. [ABSTRACT FROM AUTHOR]