Antidepressants and anxiolytics in aquatic environments as emerging contaminants and their role in antibiotic resistance

The increasing occurrence of emerging contaminants (ECs) in aquatic ecosystems, particularly non-antibiotic drugs such as antidepressants and anxiolytics, has raised global concern. These compounds are continuously released into the environment through human excretion, inefficient wastewater treatment plants, and improper disposal. Although widely detected across regions of the world, their ecological relevance has been neglected because they occur at trace concentrations (ng/L). This review compiles recent data on the occurrence, environmental distribution, and biological effects of antidepressants and anxiolytics, and their metabolites in aquatic systems, with a focus on potential impacts on bacterial communities and the development of antimicrobial resistance. Reported environmental concentrations reached up to 490 ng/L for diazepam and 3040 ng/L for venlafaxine. In addition to the ecotoxicological effects widely described in aquatic organisms, recent evidence suggests that these pharmaceuticals can also alter bacterial physiology and trigger cellular stress responses even at trace concentrations. While impacts on aquatic animals are well characterized, effects on bacterial communities remain a frontier of knowledge. Depending on exposure conditions, these compounds have been associated with phenotypic and genotypic effects, including increased production of reactive oxygen species, modulation of cell membrane permeability, activation of multidrug efflux pumps, downregulation of porins, alterations in gene expression, and increased horizontal gene transfer. These effects suggest a still underestimated role of these non-antibiotic drugs in the selection and dissemination of antibiotic resistance in aquatic environments. It is important to highlight that the compiled evidence reveals marked geographical asymmetries in monitoring efforts. In many countries, the scarcity of recent data prevents robust conclusions, making it uncertain whether the apparent absence of these compounds actually reflects low environmental occurrence or instead results from a lack of systematic measurements and reporting in the literature. Filling this gap is essential to avoid underestimating exposure and the associated ecological and public health risks.