Structural dependence of pharmaceutical transformation and toxicity evolution during water oxidation and disinfection

Pharmaceuticals are contaminants of emerging concern in drinking water sources due to continuous discharge and incomplete removal. Oxidation and disinfection, including chlorination, ozonation, and UV-based advanced oxidation, can generate transformation products that retain or acquire biological activity, making parent removal an insufficient indicator of detoxification. Organized around reactive molecular moieties, this review critically synthesizes the occurrence, transformation kinetics, pathways, and toxicity evolution of selected pharmaceuticals. Reported transformations reflect the combined influence of molecular structure, scaffold context, and treatment conditions. Recurring reactions include heterocycle oxidation and ring opening, amine N-oxidation and N-dealkylation, olefinic oxidation and cleavage, phenolic hydroxylation, quinone formation and halogenation, and sulfur oxidation and C-S bond cleavage. These reactions can retain, disrupt, or generate toxicity-relevant molecular features, with effects depending on product identity and biological endpoints. Product-specific bioassays are distinguished from reaction-mixture effects, computational predictions, and indirect estimates to clarify the evidence supporting structure-toxicity relationships. Differences in treatment conditions and analytical coverage constrain cross-study attribution of pathway differences to molecular structure. Future studies should integrate time-resolved non-target analysis, multi-endpoint toxicity testing, and condition-aware machine learning supported by standardized experimental data. This structure-oriented synthesis identifies priorities for product confirmation and toxicity-informed evaluation of drinking-water treatment.