Dexamethasone is used to treat conditions such as arthritis, blood/hormone disorders, allergic reactions, 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).Dexamethasone belongs to a class of drugs known as corticosteroids. It decreases your immune system's response to various diseases to reduce symptoms such as swelling and allergic-type reactions.