A radical (often, but unnecessarily called a free radical) is an atom or group of atoms that have one or more unpaired electrons. A prominent feature of radicals is that they have extremely high chemical reactivity, which explains not only their normal biological activities, but how they inflict damage on cells. The free radical formation in a living system leads to oxidative damage of macromolecules, such as DNA, proteins and lipids. Oxidative stress is defined most simply as the imbalance between the production of free radicals and the body’s antioxidant defense. Propofol (2,6-diisopropylphenol) is a versatile, short-acting, intravenous (i.v.)sedative-hypnotic agent initially marketed as an anesthetic, and now also widely used for the sedation of patients in the intensive care unit (ICU). Propofol's structure contains a phenolic hydroxyl group and thus resembles that of a-tocopherol (vitamin E), a natural antioxidant. As shown by both in vitro and in vivo studies, the antioxidant activity of propofol results partly from this phenolic chemical structure. Propofol has been reported to inhibit lipid peroxidation in various experimental models to protect cells against oxidative stress and to increase the antioxidant capacity of plasma in humans.