Rationale The presence of pharmaceuticals in water is a worldwide concern due to potential damage to human and environmental health. For example, compounds such as the β‐blocker atenolol (ATE), widely used for the treatment of cardiac disease, are detected in drinking water since conventional water treatment plants are not designed to remove them. Thus, the evaluation of ATE removal at different water oxidative treatment processes, identification of its degradation products and evaluation of their toxicity is necessary. Methods Aqueous solutions of ATE (10 mg/L) were submitted to oxidative treatments of chlorination ([NaClO] = 10 mg/L), ozonation ([O3] = 8 mg/L), photocatalysis ([TiO2] = 120 mg/L and UV‐C light) and photolysis (UV‐C light). The removal of ATE and formation of degradation products (DPs) were monitored by mass spectrometry. To assess acute cytotoxicity, DPs were submitted to colorimetric MTT assay using HepG2 cells. The Ecological Structure Activity Relationships (ECOSAR) software was applied to estimate the acute and chronic toxicity of identified DPs at different trophic levels. Results Photocatalysis was the treatment that demonstrated greater efficiency, removing 94% of the initial ATE. For the four tested treatments, 12 DPs were confirmed after 30 min. Moreover, some of the identified DPs were unpublished in the literature. Through high‐resolution mass spectrometry (HRMS), it was possible to elucidate the structure of the DPs. Solutions of DPs were not considered to be toxic to HepG2 cells. Only the DP with a molecular formula of C13H19NO3 (m/z 238.1438) could be considered detrimental to daphnid and green algae. Conclusions Low rates of organic matter removal and high rates of ATE degradation were obtained in the applied treatments after 30 min. Although the treated solutions were not toxic to HepG2 cells, one of the degradation products can be considered an environmental concern since it presents chronic toxicity to daphnid and green algae.

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