Which compound is the main active ingredient in many antacid tablets and has the formula Mg(OH)₂?

Magnesium hydroxide is the compound with the formula Mg(OH)₂ used as an active ingredient in many antacid tablets.

Magnesium hydroxide is a sparingly soluble inorganic compound. Its suspension in water is commonly called milk of magnesia, but this question concerns the chemical substance itself rather than the familiar product name. Because it reacts with excess stomach acid, it can relieve heartburn and indigestion.

The compound is also used as a saline laxative. Its effects depend on dose, formulation, and individual health factors, so medicines should be used according to their labels or professional advice. Magnesium hydroxide’s low solubility means it behaves differently from highly soluble strong bases such as sodium hydroxide.

It is often confused with magnesium carbonate, another magnesium compound found in some antacid products. Aluminum hydroxide is also used in antacids, but it has a different formula and can have different digestive effects. The magnesium ion in magnesium hydroxide is Mg²⁺, balanced by two hydroxide ions.