Vitamin and Mineral Supplement. Magnesium Oxide 400 mg Tablets. Description: Each tablet contains: Magnesium Oxide [Equivalent to 241.3 mg of elemental magnesium (19.86 mEq)] 400 mg. Magnesium Oxide Tablets 400mg per serving. Concentrated form of magnesium for optimal absorption. Magnesium is essential for healthy heart, muscle and nervous system function and aids in the body's absorption and utlization of other nutrients including calcium and phosphorous. Magnesium is also believed to aid in abating leg and muscle cramps. Image shown is for display purposes only. Actual package varies.