The shortcomings of conventional non-biological synthesis methods like generation of toxic byproducts, energy consumptions, and involved cost have shifted the attention towards green syntheses and other alternative methods. Among metallic NPs, gold nanoparticles (AuNPs), silver (AgNPs), cupper (CuNPs) and iron oxides IONPs are the most extensively used NPs, owing to their unique physicochemical properties. These nanoparticles have potential therapeutic applications, as those are synthesized with biomolecules as reducing and stabilizing agent(s). The new methods of NPs synthesis are simple, eco-friendly, and cost-effective with the use of renewable energy sources; however, their toxicity is not well-know yet. Although thousands of different nanoparticles (NPs) have been identified and synthesized to date, well-defined, consistent guidelines to control their exposure and evaluate their potential toxicity have yet to be fully established. As potential applications of nanotechnology in numerous fields multiply, there is an increased awareness of the issue of nanomaterials’ toxicity among scientists and producers managing them. An updated inventory of customer products containing NPs estimates that they currently number over 5.000 products; ten years ago, they were a fifth of this. More often than not, products bear no information regarding the presence of NPs in the indicated list of ingredients or components. Consumers are therefore largely unaware of the extent to which nanomaterials have entered our lives, let alone their potential risks. The release of particularly reactive metal ions capable of crossing cell membranes has been implicated in NPs toxicity as well as reactive oxygen species production. In this overview we discuss the origin, behavior and biological toxicity of different metal considering their increasingly widespread use.