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Wastewater-based epidemiology (WBE) includes the analysis of human metabolic biomarkers of xenobiotics in influent wastewater. WBE complements existing drug utilization approaches and provides objective, spatio-temporal information on the consumption of pharmaceuticals in the general population. This approach was applied to 24-h composite influent wastewater samples from Leuven, Belgium. Daily samples were analysed from September 2019 to December 2019 (n = 76), and on three days of the week (Monday, Wednesday, Saturday) from January 2020 to April 2022 (n = 367). Sample analysis consisted of 96-well solid-phase extraction and liquid chromatography coupled to tandem mass spectrometry. Measured concentrations of 21 biomarkers for antidepressant and opioid use were converted to population-normalized mass loads (PNML) by considering the flow rate and catchment population. To capture population movements, mobile phone data was used. Amitriptyline, hydroxy-bupropion, norcitalopram, citalopram, normirtazapine, trazodone, O-desmethylvenlafaxine, codeine, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), methadone, morphine, O-desmethyltramadol, and tramadol were included in the temporal assessment since concentrations were above the lower limit of quantification. The PNML of most biomarkers increased (with 3-119 %) throughout the sampling period. The population disruption during the COVID-19 pandemic led to a major change in the socio-demographics of the catchment area, resulting in temporal differences in the PNML of the different biomarkers. As such, higher PNML were observed during the different lockdown phases, which were characterized by the outflow of university students and a decreasing commuting in and out the catchment area. The effects of the fluctuating socio-demographics of the catchment population were further evidenced by the different week-weekend pattern of PNMLs over the course of the sampling campaign. Mean parent/metabolite ratios (i.e., citalopram/norcitalopram, tramadol/O-desmethyltramadol, venlafaxine/O-desmethylvenlafaxine, and methadone/EDDP) remained relatively stable throughout the entire sampling campaign (RSD% below 25 % for all ratios, except for methadone/EDDP) and therefore were not affected by this population change., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Introduction: Pharmaceutical reimbursement policies should aim for satisfactory health care at reasonable prices and assure treatment adherence while avoiding wasteful pharmaceutical spending. In Belgium, the maximum billing system, also called "Maximum Amount Fixed" system (MAF), ensures that out-of-pocket payments beyond a ceiling are fully reimbursed to guarantee pharmaceutical treatment in vulnerable population groups (e.g. low-income people and chronic patients). Starting from 2015, a policy change occurred and these expenses were waived during the same calendar year once this ceiling was reached instead of reimbursing these expenses in the next fiscal year. Each subsequent fiscal year, out-of-pocket payments were reintroduced in January., Methods: Longitudinal dispensing trends, from early 2013 to mid-2020, for 13 prevalent reimbursed opioids were investigated., Results: For MAF patients, significant seasonal increases in opioid dispensing reoccurred towards the end of the year. This pattern was absent for non-MAF patients and was only observed after the 2015 policy change. Periodic changes in out-of-pocket expenses, a characteristic of the reimbursement policy in Belgium, clearly influences opioid availability in an already at-risk group., Conclusions: Out-of-pocket reimbursement policies should be urgently re-evaluated to minimize unnecessary opioid exposure, while preserving the affordability of pain treatment for vulnerable patients., Competing Interests: Declarations of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)