Wastewater is a significant source of environmental contamination, introducing a wide array of chemicals, including endocrine disruptors, which pose substantial health risks to living organisms. To mitigate these dangers, sewage treatment plants (WWTPs) are employed to improve water quality before discharge into the environment. However, conventional wastewater treatment processes often fail to adequately remove all contaminants. This study evaluates the effectiveness of the Guelma WWTP by assessing the cytotoxicity of water samples collected from three stages of the treatment process: pretreatment (S1), decantation (S2), and disinfection (S3). The Allium cepa test was used as a bioindicator, where onion bulbs were exposed to the water samples for 24, 48, and 72 hours, followed by macroscopic analysis. The results indicated a significant reduction in root growth across all samples, with the most pronounced inhibition observed in S1, showing a 50% inhibition rate. This suggests a higher cytotoxicity in the untreated wastewater compared to the treated samples (S2 and S3). Despite some reduction in pollutant load through the treatment process, the residual cytotoxicity in S3 indicates that the current treatment methods at Guelma WWTP may not be fully effective. In conclusion, while the treatment process at Guelma WWTP reduces cytotoxicity, the persistence of harmful effects underscores the need for more efficient treatment alternatives to enhance pollutant removal and safeguard environmental health.