Magnesium Oxide Supplements is used to prevent and treat low amounts of magnesium in the blood. Magnesium is very important for the normal functioning of cells, nerves, muscles, bones, and the heart. Usually, a well-balanced diet provides normal blood levels of magnesium. However, certain situations cause your body to lose magnesium faster than you can replace it from your diet. These situations include a poor diet, alcoholism, or other medical conditions (e.g., severe diarrhea/vomiting, stomach/intestinal absorption problems, poorly controlled diabetes).