Tscharke, B.J.O'Brien, J.W.Ahmed, F.Nguyen, L.Ghetia, M.Chan, G.Thai, P.Gerber, C.Bade, R.Mueller, J.Thomas, K.V.White, J.Hall, W.2025-12-182025-12-182023Addiction, 2023; 118(3):480-4880965-21401360-0443https://hdl.handle.net/11541.2/32343Data source: Supplementary information, https://doi.org/10.1111/add.16083<h4>Background and aim</h4>From 1 February 2018, codeine was rescheduled from an over-the-counter (OTC) to a prescription-only medicine in Australia. We used wastewater-based epidemiology to measure changes in population codeine consumption before and after rescheduling.<h4>Methods</h4>We analysed 3703 wastewater samples from 48 wastewater treatment plants, taken between August 2016 and August 2019. Our samples represented 10.6 million people, 45% of the Australian population in state capitals and regional areas in each state or territory. Codeine concentrations were determined by liquid chromatography-tandem mass spectrometry and converted to per-capita consumption estimates using the site daily wastewater volume, catchment populations and codeine excretion kinetics.<h4>Results</h4>Average per-capita consumption of codeine decreased by 37% nationally immediately after the rescheduling in February 2018 [95% confidence interval (CI) = 35.3-39.4%] and substantially in all states between 24 and 51% (95% CI = 22.4-27.0% and 41.8-59.4%). The decrease was sustained at the lower level to August 2019. Locations with least pharmacy access decreased by 51% (95% CI = 41.7-61.7%), a greater decrease than 37% observed for those with greater pharmacy access (95% CI = 35.1-39.4%). Regional areas decreased by a smaller margin to cities (32 versus 38%, 95% CI = 30.2-34.1% versus 34.9-40.4%, respectively) from a base per-capita usage approximately 40% higher than cities.<h4>Conclusion</h4>Wastewater analysis shows that codeine consumption in Australia decreased by approximately 37% following its rescheduling as a prescription-only medicine in 2018. Wastewater-based epidemiology can be used to evaluate changes in population pharmaceutical consumption in responses to changes in drug scheduling.enCopyright 2022 The Authors. Addiction published by John Wiley & Sons Ltd on behalf of Society for the Study of Addiction. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. (http://creativecommons.org/licenses/by-nc-nd/4.0/)HumansCodeineAnalgesics, OpioidPharmaciesAustraliaNonprescription DrugsWastewaterJournal article10.1111/add.16083000892041100001Ghetia, M. [0000-0001-6340-9354]