Continuously increasing consumption of antibiotics in health care has led to increased concentration of these compounds in surface water through wastewater treatment systems, which in turn cause adverse effects on the aquatic ecosystems of the receiving water bodies because of the intrinsic biological activity of these compounds. However, there are limited efforts on the remediation of water pollution because of antibiotics using an effective and clean technology. In this study, the photocatalytic activity of TiO₂, CdS, and ZnS semiconductor nanoparticles was employed to degrade the metronidazole antibiotic using visible light irradiation. These particles were also modified by silver doping to improve their degradation efficiency. Note that doping with silver greatly enhanced the degradation efficiency of these nanoparticles. The specific concentration of silver dopants necessary to achieve the maximum degradation efficiency was also identified for TiO₂, CdS, and ZnS nanoparticles. The basic mechanism of doping and the photocatalytic processes were explored in detail. A kinetic study of the degradation reaction fits first order kinetics well for all three cases. The reusability and stability of these photocatalysts was confirmed by cyclic degradation tests.