While iron oxide used to make nanoparticles is biodegradable, the toxicity of magnetic nanoparticles is still under investigation. Some research has found no signs of damage to cells, while others claim that small (< 2 nm) nanoparticles can diffuse across cell membranes and disrupt organelles.
In addition, very high concentrations of iron oxide can disrupt homeostasis and lead to iron overload, which can damage or alter DNA, affect cellular responses, and kill cells. Lysosymes can also digest the nanoparticles and release free iron which can react with hydrogen peroxide to form free radicals, leading to cytotoxic, mutagenic, and carcinogenic effects.