 A study found that patients with chronic obstructive pulmonary disease (COPD) had a decrease in oxidative stress, including glutathione oxidation and lipid peroxidation, when xanthine oxidase was inhibited using allopurinol.Oxidative stress can be caused by hydroxyl free radicals and hydrogen peroxide, both of which are byproducts of XO activity.Increased concentration of serum uric acid has been under research as an indicator for cardiovascular health factors, and has been used to strongly predict mortality, heart transplant, and more in patients.