Milk of magnesia works in two ways: it draws water into the intestines to soften stool and stimulate a bowel movement, and it neutralizes stomach acid on contact. The active ingredient, magnesium hydroxide, is behind both effects. Which one you experience depends on the dose you take.
The Laxative Effect
When you swallow milk of magnesia for constipation, the magnesium hydroxide travels to your intestines, where it pulls water from surrounding tissues through a process called osmosis. The extra water softens hard stool and increases the volume of material in your bowel. That added bulk stretches the intestinal walls, which triggers the natural muscular contractions that push everything along. The result is a bowel movement, typically within 30 minutes to 6 hours of taking a dose.
This osmotic action is what sets milk of magnesia apart from stimulant laxatives, which work by directly irritating the intestinal lining to force contractions. Because milk of magnesia relies on water movement instead, it tends to produce a softer, more natural stool. Drinking a full 8-ounce glass of liquid with each dose helps the process work properly and reduces the chance of dehydration.
The Antacid Effect
At lower doses, milk of magnesia acts as an antacid. Magnesium hydroxide reacts with the hydrochloric acid in your stomach, converting it into water and dissolved magnesium. This chemical reaction directly raises the pH in your stomach, reducing the burning sensation of heartburn or indigestion.
The neutralization doesn’t happen all at once. Magnesium hydroxide is a suspension, meaning the solid particles are mixed into liquid rather than fully dissolved. As stomach acid breaks down the particles that have already dissolved, more of the solid gradually dissolves and continues neutralizing acid. This slow-release quality gives it a longer-lasting effect than some other antacids that dissolve immediately and wear off quickly.
Dosing for Constipation
The standard adult dose for constipation is 30 to 60 mL (2 to 4 tablespoons). Children aged 6 to 11 take 15 to 30 mL (1 to 2 tablespoons). Children under 6 should not take it without a doctor’s guidance. The maximum for adults is 60 mL in a 24-hour period.
You can take the full dose at once, preferably at bedtime, or split it into smaller doses throughout the day. Shaking the bottle well before measuring is important because the magnesium hydroxide settles at the bottom over time, and an unshaken dose may be weaker or stronger than intended.
How It Compares to Magnesium Citrate
Both milk of magnesia and magnesium citrate are osmotic laxatives that pull water into the intestines. The key difference is versatility and potency. Magnesium citrate is used only as a laxative, often for bowel preparation before medical procedures, and tends to produce a more aggressive effect. Milk of magnesia is gentler and doubles as an antacid for heartburn and indigestion, making it a more practical option for everyday use.
Side Effects Worth Knowing
The most common side effects are cramping, loose stools, and mild nausea. These are typically signs that the osmotic effect is working a bit too aggressively, and they often resolve by lowering the dose. Diarrhea from overuse can lead to dehydration, so staying hydrated matters.
The more serious concern is a buildup of magnesium in the blood, a condition called hypermagnesemia. Healthy kidneys handle excess magnesium easily by filtering it out, so this is rare in people with normal kidney function. The warning signs include confusion, drowsiness, facial flushing, muscle weakness, and irregular heartbeat. If you notice any of these, stop taking it.
Why Kidney Function Matters
Your kidneys are responsible for clearing excess magnesium from your body. When kidney function drops below roughly 20 to 30 percent of normal capacity, this filtering system can’t keep up. Magnesium levels in the blood start climbing, and the risk of hypermagnesemia rises significantly. People with advanced chronic kidney disease are especially vulnerable, because even small amounts of magnesium from an antacid or laxative can push blood levels into a dangerous range. If you have kidney disease, milk of magnesia is generally not safe to use without medical supervision.
Getting the Best Results
Milk of magnesia works best as a short-term solution. For occasional constipation, a single bedtime dose with a full glass of water will usually produce a bowel movement by morning. For heartburn, a smaller dose taken after meals or at the onset of symptoms neutralizes acid within minutes.
If you’re using it regularly for more than a week, that’s a sign the underlying issue needs attention rather than ongoing symptom management. Chronic constipation and frequent heartburn both have treatable causes, and relying on milk of magnesia long-term increases the chance of electrolyte imbalances and dependence on laxatives for normal bowel function.