Peroxides produced in the presence of a free radical by the oxidation of unsaturated fatty acids in the cell in the presence of molecular oxygen. The formation of lipid peroxides results in the destruction of the original lipid leading to the loss of integrity of the membranes. They therefore cause a variety of toxic effects in vivo and their formation is considered a pathological process in biological systems. Their formation can be inhibited by antioxidants, such as vitamin E, structural separation or low oxygen tension.

(lipid hydroperoxide) Highly reactive, non-radical peroxols comprised of a fatty acid modified by a hydroperoxide group (-OOH) that are produced by lipid peroxidation. Lipid hydroperoxides are formed by sequential abstraction of allylic hydrogens from unsaturated fatty acids, which occurs in the presence of reactive oxygen species. These modified lipids can be further modified to create cytotoxic reactive aldehydes, such as malondialdehyde (MDA) and 4-hydroxy-2-nonenal (HNE). Lipid hydroperoxide levels can be measured to assess the extent of oxidative stress.