Magnetic nanoparticles (MNPs) are engineered materials that can be guided by an external magnetic field and functionalized for biomedical applications. They have been used in magnetic resonance imaging (MRI), targeted drug and gene delivery, magnetic hyperthermia, tissue engineering, cell tracking, and bioseparation. MNPs can also be used in theragnostics, combining therapeutic and diagnostic functions. However, their potential cytotoxicity arises from their enhanced reactive surface area, ability to cross cell and tissue barriers, and resistance to biodegradation. Oxidative stress, a key aspect of nanotoxicity, manifests in three tiers: activation of reactive oxygen species (ROS), pro-inflammatory response, and DNA damage leading to apoptosis and mutagenesis. In vivo, MNPs are quickly recognized and internalized by macrophages of the reticuloendothelial system (RES), which can neutralize their toxicity but also reduce their circulation time. The size, composition, and surface chemistry of MNPs influence their intracellular uptake, biodistribution, macrophage recognition, and cytotoxicity. Current studies on MNP toxicity highlight the importance of surface chemistry in minimizing nanotoxicity and optimizing MNPs for biomedical use. Iron oxide MNPs, such as magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), are the most commonly used due to their biocompatibility and stability. Surface coatings, such as dextran, polyethylene glycol (PEG), and silica, are used to improve MNP stability, dispersibility, and biocompatibility. Functionalization with peptides, nucleic acids, and antibodies allows MNPs to target specific cells and tissues. MNPs have been used in magnetic hyperthermia, MRI, bioseparation, and targeted drug delivery. However, their potential toxicity, including oxidative stress and cytotoxicity, must be carefully evaluated. The development of MNPs for biomedical applications requires a balance between their functional capabilities and their safety profile. The field of magnetic nanotoxicology is rapidly evolving, with ongoing research aimed at understanding the toxic effects of MNPs and developing strategies to minimize their risks. The integration of bioengineering, biomedical, and toxicology disciplines is essential for advancing the safe and effective use of MNPs in medicine.Magnetic nanoparticles (MNPs) are engineered materials that can be guided by an external magnetic field and functionalized for biomedical applications. They have been used in magnetic resonance imaging (MRI), targeted drug and gene delivery, magnetic hyperthermia, tissue engineering, cell tracking, and bioseparation. MNPs can also be used in theragnostics, combining therapeutic and diagnostic functions. However, their potential cytotoxicity arises from their enhanced reactive surface area, ability to cross cell and tissue barriers, and resistance to biodegradation. Oxidative stress, a key aspect of nanotoxicity, manifests in three tiers: activation of reactive oxygen species (ROS), pro-inflammatory response, and DNA damage leading to apoptosis and mutagenesis. In vivo, MNPs are quickly recognized and internalized by macrophages of the reticuloendothelial system (RES), which can neutralize their toxicity but also reduce their circulation time. The size, composition, and surface chemistry of MNPs influence their intracellular uptake, biodistribution, macrophage recognition, and cytotoxicity. Current studies on MNP toxicity highlight the importance of surface chemistry in minimizing nanotoxicity and optimizing MNPs for biomedical use. Iron oxide MNPs, such as magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃), are the most commonly used due to their biocompatibility and stability. Surface coatings, such as dextran, polyethylene glycol (PEG), and silica, are used to improve MNP stability, dispersibility, and biocompatibility. Functionalization with peptides, nucleic acids, and antibodies allows MNPs to target specific cells and tissues. MNPs have been used in magnetic hyperthermia, MRI, bioseparation, and targeted drug delivery. However, their potential toxicity, including oxidative stress and cytotoxicity, must be carefully evaluated. The development of MNPs for biomedical applications requires a balance between their functional capabilities and their safety profile. The field of magnetic nanotoxicology is rapidly evolving, with ongoing research aimed at understanding the toxic effects of MNPs and developing strategies to minimize their risks. The integration of bioengineering, biomedical, and toxicology disciplines is essential for advancing the safe and effective use of MNPs in medicine.