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. Magnesium is also essential in the formation of bones and teeth and in converting blood sugar into energy. Helps Nerve and Muscle Function. Helps protect cardiovascular system.