Dexamethasone is a corticosteroid (cortisone-like medicine or steroid). It decreases your immune system's response to various diseases to reduce symptoms such as swelling and allergic reactions. Dexamethasone is used to treat conditions such as allergic reactions, arthritis, blood/hormone disorders, skin diseases, eye problems, breathing problems, bowel disorders, cancer, and immune system disorders. It is also used as a test for an adrenal gland disorder (Cushing's syndrome).