The mineral supplement is used to manage or balance low amounts of magnesium in the blood. Magnesium supports normal functioning of cell, nerves, muscles, bones and heart. Usually, a well - balanced diet provides normal blood levels of magnesium. Magnesium oxide is a mineral supplement used to treat or prevent low levels of magnesium in the blood. Magnesium is important for many systems in the body, but it is especially vital for healthy muscles and bones. Some people use magnesium oxide to relieve heartburn or indigestion.