This study conducted an acute toxicity assessment using Daphnia magna to evaluate the potential for increased toxicity when pharmaceuticals persist in aquatic environments not as single substances but in mixed forms. In single-substance toxicity tests, the antibiotics clarithromycin and sulfamethoxazole showed EC50 values of 22.3 mg L-1 and 61.05 mg L-1, respectively. However, the EC50 for the mixture of the two substances was determined to be 31.1 mg L-1. Based on these findings, applying the Similar Mode of Action (MOA) equation from the QSAR Toolbox, as recommended by OECD non-testing methods, produced an estimated EC50 of 33.7 mg L-1 for the mixture, showing a difference of 8.5% compared to the experimental value. This study confirms that combined exposure to pharmaceuticals can increase toxicity due to synergistic effects, indicating a significant potential risk to aquatic ecosystems. According to the UN-GHS classification criteria, clarithromycin, sulfamethoxazole, and their mixture were all classified as Category 3, indicating potential toxicity to aquatic organisms. These results emphasize the importance of toxicity assessments that consider interactions between multiple contaminants in real environmental settings, contributing to the development of effective toxicity eva...