Dexamethasone is a glucocorticosteroid, a man-made drug meant to mimic a natural hormone produced in our adrenal glands. It is prescribed under various names, including Decadron, Dexamethasone Intensol and dexamethasone sodium phosphate.
When used in the course of treatment for cancer, dexamethasone can have one or more purposes:
Dexamethasone is prescribed by doctors for a host of other reasons that have little or nothing to do with cancer, and therefore won't be listed here.
In all honesty, your guess is probably as good as anyone else's guess. Like many drugs that are used every day, the exact mechanism of action of dexamethasone is not understood. Doctors and health care professionals just know that it works in certain situations for certain things. That doesn't however mean there aren't some moderately supported hypotheses about how it works.
For instance, since it is a steroid, dexamethasone appears to have anti-inflammatory properties; it stops white blood cells from rushing towards tumors and creating problems with swelling.
It also seems to be effective in the treatment of blood cancers but inducing apoptosis—cell death—in cancerous leukocytes.
But as for how it helps with one's appetite, or how it manages to suppress the urge to vomit after receiving chemotherapy drugs, that much remains unknown.
As a drug, dexamethasone is an old-timer. A true work-horse. It has been on the market and FDA approved since at least 1984, and even if its mechanism of action is not well-understood, its side effect profile is. To go along with many possible side effects of taking dexamethasone, there are several serious precautions and possibly drug reactions to consider before beginning taking this drug.
These are things that one's doctor and/or pharmacist will discuss with all patients before putting them on a course of treatment that includes dexamethasone. To learn more about these potential adverse effects, you can visit the MedlinePlus page on dexamethasone by the National Institutes of Health.