Antacid-laxative for the relief of heartburn, sour stomach, acid indigestion, and mild constipation. Each teaspoon contains 400 mg of magnesium hydroxide. Magnesium hydroxide is used as an antacid to neutralize stomach acid and also as a laxative. In some cases, it is prescribed to help relieve mild colic. Magnesium hydroxide will reduce stomach acidity for a short period of time.