Water-soluble polymers (WSPs) are widely used in industrial, medical, and consumer applications. Their large-scale production and the lack of dedicated regulatory frameworks contribute to their continuous and uncontrolled release into aquatic environments. Although WSPs have been traditionally considered environmentally safe based on conventional apical (eco)toxicological assays, growing evidence indicates that they may induce sub-lethal effects in aquatic organisms. So far, most studies have focused on pure polymers, overlooking the complexity of commercial formulations actually entering the environment. Among WSPs, polyethylene-glycol (PEG) is one of the most widely used, particularly in pharmaceutical products, such as laxatives, often composed of high-molecular-weight PEG and formulation additives that may significantly alter environmental fate and (eco)toxicity. This study aims to investigate the potential effects of two formulations of the same PEG-based laxative: (i) pure PEG-4000, (ii) PEG-4000 (5 g in 10 mL) dissolved in purified water and combined with two additives (potassium sorbate, citric acid). Danio rerio embryos were exposed to both treatments at equivalent PEG concentration of 0.1 mg/L. A multi-tier approach was applied to assess effects from molecular to organism level, including metabolomics, biomarkers of oxidative stress and neurotoxicity, cardiac activity, morphometric parameters, histology, (immuno)histochemistry, spontaneous coil-tailing activity, and swimming behaviour. Results revealed formulation-dependent responses.  Embryos exposed to PEG+additives showed significantly (p < 0.05) lower reactive-oxygen species (ROS) production and ethoxyresorufin-O-deethylase (EROD) activity. Conversely, cardiac activity exhibited divergent responses, decreasing significantly in embryos exposed to pure PEG, but increasing significantly following exposure to PEG+additives. Moreover, both treatments altered swimming pattern under dark conditions and stress-related behaviour under light conditions. Although some analyses are still ongoing, these findings suggest that additives in commercial WSP-based formulations play a key role in modulating the toxicity of WSPs. Overall, this study highlights the importance of assessing complete commercial formulations to improve the environmental risk assessment of WSPs.