Magnesium oxide is a dietary supplement that is used as an antacid, a laxative or a nutritional supplement, according to MedlinePlus. Magnesium is an element that the human body must have to function in a normal capacity.
Magnesium regulates many functions in the body, such as muscle and nerve function, blood glucose control and energy production, states National Institutes of Health. Many conditions can cause high or low levels of magnesium, notes WebMD. Alcohol abuse, long-term diarrhea or diseases that affect how the intestines absorb nutrients can cause low magnesium levels. Dehydration, kidney failure or an underactive thyroid gland may cause high magnesium levels.