Magnesium hydroxide is an inorganic compound. It is naturally found as the mineral brucite. Magnesium hydroxide can be used as an antacid or a laxative in either an oral liquid suspension or chewable tablet form. Additionally, magnesium hydroxide has smoke suppressing and flame retardant properties and is thus used commercially as a fire retardant. It can also be used topically as a deodorant or for the relief of canker sores (aphthous ulcers).

Magnesium hydroxide is second only to aluminum hydroxide as the most widely used antacid active agent. This compound is commonly used in combination with aluminum hydroxide in suspensions and powder formulations. It has a high acid binding capacity and reacts rapidly to neutralize stomach acids.