Skilled nurse instructed patient about medication Dexamethasone is a corticosteroid that prevents the release of substances in the body that cause inflammation. Dexamethasone is also used to treat certain types of cancer. Your doctor may instruct you to follow a low-sodium, low-salt, potassium-rich, or high-protein diet. Follow these directions. Dexamethasone may cause an upset stomach. Take dexamethasone with food or milk. Dexamethasone may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away: upset stomach, stomach irritation, vomiting, headache, dizziness.