Licit and illicit drug consumption in the Paris conurbation: assessing short to mid-term temporal trends through WBE and comparison with official statistics

Wastewater-based epidemiology (WBE) is increasingly promoted as a tool for monitoring drug use, yet its added value for drug policy remains constrained by methodological uncertainties and limited integration with conventional indicators. This study evaluated the extent to which WBE can reliably characterize temporal drug consumption patterns and complement established surveillance approaches. A two-year campaign (March 2023-March 2025) was conducted at a wastewater treatment plant serving approximately 2.3 million inhabitants of the Paris conurbation. Bi-weekly composite samples (n = 202) were analyzed by LC-MS/MS for major licit and illicit drugs. Consumption was estimated using de facto population-normalized daily intakes, and temporal variations were explored using non-parametric statistics. Results confirm established consumption hierarchies (cannabis > cocaine > MDMA > ketamine, amphetamines-like stimulants) and typical recreational patterns, with pronounced weekend increases for MDMA, cocaine and alcohol, while cannabis exhibited stable use, suggesting its integration into routine consumption. Holiday periods were associated with a redistribution of consumption across the week rather than a simple overall increase, highlighting the importance of both temporal resolution and accurate assessment of the contributing population when interpreting consumption trends. While WBE-derived estimates for alcohol and tobacco were consistent with national sales data, reinforcing its validity for licit substances, the analysis also underscores persistent limitations for illicit drugs, particularly cannabis, for which prevalence estimates varied widely depending on normalization assumptions. A refined calculation framework yielded values closer to epidemiological benchmarks but remains dependent on contested parameters. Overall, these findings suggest that WBE is better suited to capturing relative changes and temporal dynamics than absolute prevalence, and that its policy relevance lies primarily in its ability to identify complement monitoring systems, rather than to replace established monitoring tools.