Wikipedia
Pro-oxidants are chemicals that induce oxidative stress, either by generating reactive oxygen species or by inhibiting antioxidant systems. The oxidative stress produced by these chemicals can damage cells and tissues, for example an overdose of the analgesic paracetamol (acetaminophen) can fatally damage the liver, partly through its production of reactive oxygen species.
Some substances can serve as either antioxidants or pro-oxidants, depending on conditions. Some of the conditions that are important include the concentration of the chemical and if oxygen or transition metals are present. While thermodynamically very favored, reduction of molecular oxygen or peroxide to superoxide or hydroxyl radical is fortunately spin forbidden. This greatly reduces the rates of these reactions, thus allowing aerobic life to exist. As a result, the reduction of oxygen typically involves either the initial formation of singlet oxygen, or spin-orbit coupling through a reduction of a transition-series metal such as manganese, iron, or copper. This reduced metal then transfers the single electron to molecular oxygen or peroxide.