BACKGROUND AND AIM: From the 1 of 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.

METHODS: We analysed 3,703 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.

RESULTS: Average per-capita consumption of codeine decreased by 37% nationally immediately after the rescheduling in February 2018 and substantially in all states between 24% and 51%. The decrease was sustained at the lower level to August 2019. Locations with least pharmacy access decreased by 51%, a greater decrease than 37% observed for those with greater pharmacy access. Regional areas decreased by a smaller margin to cities from a base per-capita usage approximately 40% higher than cities.

CONCLUSION: 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.