The toxicity of copper oxide nanoparticles (CuO NPs) has become a significant concern. There is a growing need for educational investigations on exposure to nanomaterials, including CuO NPs, to assess their potential effects. Environmental and energy concerns have hindered societal progress, while nanoscience and nanotechnology have advanced rapidly, leading to the extensive use of nanosized metal and metal oxide particles. CuO NPs, in particular, have gained attention due to their unique properties and widespread applications, which pose risks to organisms and the environment. This review examines the toxicity of CuO NPs, focusing on their synthesis methods, modes of toxicity, and human exposure. We conducted a literature review using PubMed, Google Scholar, and Science Direct to access recent and comprehensive materials. CuO NPs exposure causes cytotoxicity, genotoxicity, immunotoxicity, ROS generation, inflammation, and oxidative stress. The toxicity of CuO NPs is influenced by their physicochemical properties, concentration, animal models, modes of exposure, and assessment methods. This review summarizes the toxicity of CuO NPs by discussing preparation methods, underlying toxicity mechanisms, modes of exposure, and factors affecting toxicity, highlighting the need for more attention to their effects even at low concentrations.