Long-term WBE monitoring of alcohol, nicotine, and caffeine in two Spanish cities: COVID-19 impacts and beyond.

Wastewater-based epidemiology (WBE) is a valuable tool for assessing population-level consumption of addictive substances. This study presents long-term WBE monitoring (2021-2025) of alcohol, nicotine, and caffeine in Madrid and Guadalajara (Spain), including the first city-wide spatial analysis in Europe. A total of 192 composite 24-h untreated wastewater samples were collected from nine wastewater treatment plants: one in Guadalajara and eight in Madrid. Regression models were applied to assess spatial and temporal trends, as well as the impact of key events such as the COVID-19 restrictions, New Year celebrations, and weekends. Key findings show that between 2021 and 2024 alcohol and nicotine use showed a decreasing trend in both cities, while caffeine consumption remained stable with slight increases. In Madrid, average alcohol consumption declined from 27.7 to 16.3 mL/day/person and nicotine by approximately 800 mg/day/1000 person; in Guadalajara, alcohol decreased from 13.1 to 7.7 mL/day/person and nicotine 500 mg/day/1000 person. In December 2023, full city-level estimates from Madrid revealed alcohol consumption between 12.8-18.4 mL/day/person, nicotine 2180-3300 mg/day/1000 person, and caffeine 68.5-134.7 mg/day/person. Notably, alcohol and nicotine reached their highest levels among the New Year periods during the 2022-2023 celebration, the first unrestricted one after the pandemic; levels during this period dropped in the following years. These results highlight temporal and spatial patterns in the consumption of legal substances and suggest shifts in public behavior after COVID-19 and the utility of WBE as a long-term monitoring tool for public health surveillance and planning.