Dexamethasone, a synthetic adrenocortical steroid, is a white to practically white, odorless, crystalline powder. It is stable in air. It is practically insoluble in water. The molecular weight is 392.47. It is designated chemically as 9-fluoro-11 b,17,21-trihydroxy- 16a-methylpregna-1,4-diene-3,20-dione. Its chemical formula is C22H29FO5. It is about 40 times stronger than hydrocortisone.

Dexamethasone is a member of the glucocorticoid class of hormones. This means they are steroids but, unlike the anabolic steroids that we hear about regarding sports medicine, these are "catabolic" steroids. Instead of building the body up, they are designed to break down stored resources (fats, sugars and proteins) so that they may be used as fuels in times of stress.

Dexamethasone is a synthetic corticosteroid and is used to control symptoms caused by low levels of corticosteroids in the body. This can be due to many reasons, including Addison's disease or surgical removal of the adrenal gland(s). Low blood levels of corticosteroids must be treated with replacement therapy to allow the body to function normally. Corticosteroids affect the strength of heart muscle and its response to natural chemicals affecting heart rate. They affect the water and salt balance in the body and also enable the body to cope with stress. Stress includes changes in temperature, pain, fear, anxiety and illness and can be hazardous if the body is not equiped to cope, due to low levels of corticosteroids. Corticosteroids allow us to respond to stress by increasing the rate and force of the heartbeat, increasing blood supply to essential tissues (muscle, heart, brain), increasing the body's supply of energy by raising blood sugar and by several other effects on body systems.

These drugs have both strong beneficial effects and a definite potential to cause negative side effects. Dexamethasone is commonly used in both small and large animal veterinary medicine. It may be given by injection, inhalation, orally or topically. Preparations for topical use may include other active ingredients such as antibiotics, antifungals, or miticides. Dexamethasone is used therapeutically in many, mainly inflammatory and autoimmune conditions. It is also given to cancer patients undergoing chemotherapy, to counteract certain side-effects of their antitumor treatment. In certain forms of brain disease, e.g. brain tumours or secondaries, dexamethasone is used to counteract the development of edema, which would eventually compress other brain structures. It is present in certain eye drops. Dexamethasone is also used in a diagnostic context, namely in its property to suppress the natural pituitary-adrenal axis. Patients presenting with clinical signs of glucocorticoid excess (Cushing's syndrome) are generally diagnosed by a 24h urine collection for cortisol or by a Dexamethasone suppression test. During the latter, a patient takes a nighttime dose of either 1 or 4 mg of dexamethasone, and the serum cortisol levels are measured in the morning. If the levels are relatively high, then the test is positive and the patient has an autonomous source of either cortisol or ACTH, indicating Cushing's disease.

Dexamethasone is also used to decrease inflammation. It works 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, resulting in inflammation. 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. They include severe allergic reactions, inflammation of the lungs in asthma and inflammation of the joints in arthritis. Dexamethasone also decreases the numbers of white blood cells circulating in the blood. This is useful for the treatment of certain types of leukaemia, where there is an abnormally large production of certain white blood cells. Dexamethasone can be used in much higher doses than the levels of corticosteroids produced naturally by the body, and as such, the usual actions of corticosteroids become exaggerated and can be observed as side effects of this medicine.