Based on reports of ketamine seizures, self-reported consumption and harmful associated health effects, there are signs of increased ketamine use. However, monitoring population-level consumption patterns remains difficult. This study employed wastewater-based epidemiology (WBE) to address this challenge by analysing influent wastewater (IWW) from Belgium for biomarkers of ketamine consumption to evaluate spatio-temporal trends.

Daily 24-h composite IWW samples were collected from 26 locations across the northern part of Belgium (Flanders and Brussels region), every Monday and Wednesday in 2023 (n = 716). Additionally, a temporal dataset from daily IWW samples from Leuven (n = 399) and Brussels (n = 285) in 2021 and 2022 was evaluated using time series analysis. Measured concentrations of ketamine, norketamine and dehydronorketamine in IWW were transformed to population-normalised mass loads (PNML, expressed in mg/day/1000 inhabitants) to assess ketamine consumption patterns.

Ketamine, norketamine and dehydronorketamine were detected in respectively 98%, 96%, and 76% of all samples. Substantial consumption was observed in both urban and rural areas, with higher PNMLs in major cities and the east of Flanders. Temporal analysis showed no seasonal trends but an increase in weekend consumption, likely linked to recreational use. PNMLs increased 7- to 11-fold compared to available data in 2012. Actual consumption and direct disposal down the drain could be differentiated by the investigation of the ketamine to norketamine ratio.

Ketamine use is widespread across the north of Belgium, encompassing a relatively stable, year-round consumption. In future WBE studies, it is recommended to measure both norketamine and ketamine to assess ketamine consumption and to rule out any dumping events.