Egaña, IkerNogales-García, MaiteAkhrimenko, VladimirGonzález Gómez, XianaRodil Rodríguez, María del RosarioMontes Goyanes, RosaQuintana Álvarez, José BenitoMestre-Pintó, Juan IgnacioOrive, GorkaLertxundi, Unax2026-04-202026-04-202026-04-12I. Egaña et al. Tracing illicit and prescription drug use in a Spanish prison by combining wastewater analysis and pharmaceutical dispensing data. Environmental Toxicoloy and Pharmacology 124 (2026) 105021. https://doi.org/10.1016/j.etap.2026.105021https://hdl.handle.net/10347/46828Background and aim We combined wastewater-based epidemiology (WBE) and pharmaceutical dispensing records to distinguish between illicit and prescription drug use. Methods We collected 24-h composite wastewater samples during three one-week campaigns in a prison in northern Spain. Samples were analysed for pharmaceuticals, illicit drugs, nicotine and alcohol. Population-normalised daily loads (PNDLs) were converted to consumption estimates and compared with Spanish population WBE estimates and pharmacy dispensing data. Results Wastewater analysis showed high use of several pharmaceuticals, with the highest average PNDL for gabapentinoids. Pregabalin and gabapentin estimates from wastewater exceeded pharmacy dispensing, suggesting internal diversion. Cannabis consumption was about four times higher than community estimates and cocaine was also higher in all campaigns. Ketamine and MDMA use were above community medians during the February campaign and nicotine use was nearly ten times higher than community levels. Conclusion WBE identifies prescription drug diversion, illicit drug markets and quantifies licit substance use in prisons.eng© 2026 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY licenseAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/Wastewater based epidemiologyDrugs & prisonDrug traffickingGabapentinoidsIllicit drugsAlcoholjournal article10.1016/j.etap.2026.1050211872-7077open access