Dexamethasone is a type of medicine known as a corticosteroid. Corticosteroid medicines are derivatives of the corticosteroid hormones cortisol and aldosterone that are produced naturally by the adrenal glands. Dexamethasone decreases inflammation by acting within cells to prevent the release of certain chemicals that are important in the immune system. These chemicals are normally involved in producing immune and allergic responses. By decreasing the release of these chemicals in a particular area, inflammation is reduced. This can help control a wide number of disease states characterised by excessive inflammation. These include severe allergic reactions, inflammation of the lungs in asthma and inflammation of the joints in arthritis.