Milk of Magnesia" is a common name for an over-the-counter medication used to relieve constipation, indigestion, and heartburn. The active ingredient in Milk of Magnesia is magnesium hydroxide, which has several properties that make it useful for gastrointestinal issues. How It Works: Magnesium hydroxide works as an osmotic laxative, meaning it draws water into the intestines. This helps soften the stool, making it easier to pass. Additionally, magnesium hydroxide can neutralize excess stomach acid, providing relief from heartburn and indigestion.