Milk of magnesia is made of magnesium hydroxide suspended in water. The active ingredient has the chemical formula Mg(OH)â‚‚, a white, odorless mineral compound that barely dissolves in water. That poor solubility is exactly what gives the product its signature milky appearance and its name.
The Active Ingredient
Magnesium hydroxide is a crystalline solid that occurs naturally as the mineral brucite. In its pure form, it’s a fine white powder. When mixed into water, it doesn’t fully dissolve. Instead, the tiny particles stay suspended throughout the liquid, creating the opaque, milk-like consistency you see in the bottle.
A standard dose of regular-strength milk of magnesia contains 400 mg of magnesium hydroxide per 5 mL (one teaspoon). The concentrated version packs 2,400 mg into 30 mL. That’s a lot of magnesium in a small volume, which is why you only need a tablespoon or two for most uses.
Other Ingredients in the Bottle
The inactive ingredient list for the original unflavored formula is remarkably short: purified water and a trace amount of sodium hypochlorite. The sodium hypochlorite acts as a preservative to keep the suspension free of microbial growth. Flavored versions (mint, cherry, wild berry) add sweeteners and flavorings to the base formula, but the core product is essentially just magnesium hydroxide and water.
How Magnesium Hydroxide Works as a Laxative
Milk of magnesia is classified as a saline laxative. When you swallow it, the magnesium hydroxide reaches your intestines and draws water into the bowel. This extra fluid softens stool and increases its volume, which stimulates the intestinal walls to contract and move things along. Most people notice results within 30 minutes to 6 hours.
The effect is osmotic, meaning the magnesium ions pull water toward themselves through the intestinal lining. Your body absorbs only a small fraction of the magnesium, so the bulk of it stays in the gut doing its job before being eliminated.
How It Works as an Antacid
At lower doses, the same compound neutralizes stomach acid. Magnesium hydroxide is a base, so when it contacts hydrochloric acid in your stomach, it reacts to form magnesium chloride and water. This raises the pH in your stomach, relieving heartburn, acid indigestion, and sour stomach. The antacid effect kicks in faster than the laxative effect because it happens right in the stomach rather than further down the digestive tract.
Where Magnesium Hydroxide Comes From
Pharmaceutical-grade magnesium hydroxide is produced by reacting a magnesium salt (typically magnesium chloride from seawater or brine deposits) with an alkaline solution like sodium hydroxide. The reaction precipitates pure magnesium hydroxide crystals, which are then washed, dried, and milled into the fine powder used in the suspension. The process has been refined over more than a century. Charles H. Phillips patented his method for manufacturing milk of magnesia back in 1873, and the basic chemistry hasn’t changed.
Who Should Be Cautious
Because your kidneys are responsible for clearing excess magnesium from your blood, people with kidney disease need to avoid milk of magnesia or use it only under medical supervision. Healthy kidneys handle the small amount of absorbed magnesium easily, but impaired kidneys can let magnesium accumulate to dangerous levels. Signs of too much magnesium in the blood include confusion, drowsiness, muscle weakness, facial flushing, and irregular heartbeat.
For most people, milk of magnesia is safe for occasional use. It’s not designed for daily, long-term consumption as a laxative. If constipation persists beyond a week, the underlying cause is worth investigating rather than continuing to treat the symptom.