Milk of magnesia is an over-the-counter liquid medication made from magnesium hydroxide, a mineral compound suspended in water. It serves two purposes: relieving occasional constipation and neutralizing stomach acid to ease heartburn or indigestion. The milky white appearance is what gives it its name, and it has been a medicine cabinet staple for well over a century.
How It Relieves Constipation
When you swallow milk of magnesia as a laxative, the magnesium hydroxide travels to your small intestine and pulls water in through a process called osmosis. The extra fluid accumulates, stretches the intestinal walls, and triggers the natural muscle contractions that move things along. This is why it’s classified as an osmotic or “saline” laxative rather than a stimulant one. It doesn’t force your intestines to contract the way some other laxatives do. Instead, it creates the conditions for your body to do the work on its own.
The effect is relatively fast. You can expect a bowel movement anywhere from 30 minutes to six hours after taking a dose, though most people fall somewhere in between. Drinking a full 8-ounce glass of water with each dose is important, both because the medication needs fluid to work properly and because pulling water into your intestines can leave the rest of your body slightly dehydrated.
How It Works as an Antacid
At lower doses, the same compound neutralizes hydrochloric acid in your stomach. Magnesium hydroxide reacts with the acid to produce water and magnesium chloride, a harmless salt. Unlike some antacids, this reaction doesn’t generate carbon dioxide gas, so it’s less likely to cause bloating or belching. The neutralizing capacity is high, meaning a small amount can handle a significant amount of acid.
The antacid dose is much smaller than the laxative dose. For adults, it’s 1 to 3 teaspoons (5 to 15 mL) for heartburn or indigestion, compared with 2 to 4 tablespoons (30 to 60 mL) for constipation. That difference matters. If you’re taking it only for an upset stomach and accidentally use the laxative dose, you’ll likely end up with diarrhea.
Typical Dosing by Age
For constipation relief, adults and children 12 and older take 30 to 60 mL (2 to 4 tablespoons). Children ages 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 laxative dose in a 24-hour period is 60 mL for adults. You can take the full dose at once, preferably at bedtime, or split it into smaller amounts throughout the day.
For antacid use, adults and children 12 and older take 5 to 15 mL (1 to 3 teaspoons). Children under 12 should only use it under medical supervision. Always shake the bottle well before measuring, since the magnesium hydroxide settles to the bottom.
Common Side Effects
The most frequent side effects are diarrhea, stomach cramps, and nausea. These are usually mild and resolve once the medication passes through your system. Diarrhea is essentially the laxative effect working a little too well, and if it happens at a dose that was meant to be an antacid, you’ve probably taken too much.
The bigger concern with repeated use is electrolyte imbalance. Magnesium is a mineral your body carefully regulates, and flooding your system with extra magnesium can throw off that balance. This is why milk of magnesia is intended for occasional use, not as a daily laxative. If you experience diarrhea after a dose, don’t take it again without checking with a healthcare provider.
Who Should Avoid It
People with kidney problems are the group most at risk. Your kidneys are responsible for clearing excess magnesium from your blood. When kidney function is impaired, magnesium can build up to dangerously high levels, a condition called hypermagnesemia. This can affect heart rhythm and muscle function, so anyone with chronic kidney disease should steer clear of magnesium-based laxatives and antacids entirely.
People on a magnesium-restricted diet should also avoid it. During pregnancy, milk of magnesia hasn’t been formally assigned a safety category by the FDA. Animal studies haven’t shown harm, but there are no controlled human studies. It does pass into breast milk, though it hasn’t been linked to problems in nursing infants. Still, pregnant or breastfeeding women should treat it as a “when necessary” option rather than a go-to remedy.
Interactions With Other Medications
Magnesium hydroxide interacts with a surprisingly long list of other drugs. Hundreds of known interactions exist, including 14 classified as major. The core issue is that magnesium hydroxide can bind to other medications in your digestive tract, reducing how much of them your body absorbs. This is especially relevant for thyroid medications like levothyroxine, certain antibiotics, blood pressure medications, and bisphosphonates used for bone density.
Common medications that people check for interactions include metformin, gabapentin, omeprazole, lisinopril, and atorvastatin. If you take any prescription medication on a regular schedule, the simplest precaution is to separate your doses by at least two hours. Taking milk of magnesia at bedtime, well after your other medications, often solves the timing problem.
Milk of Magnesia vs. Other Laxatives
Compared to stimulant laxatives like senna or bisacodyl, milk of magnesia works through a gentler mechanism. Stimulant laxatives directly irritate the intestinal lining to force contractions, which can cause more intense cramping. Milk of magnesia simply adds fluid, letting your intestines respond naturally. On the other hand, bulk-forming laxatives like psyllium fiber work even more gently but take one to three days to produce results. Milk of magnesia sits in the middle: faster than fiber, gentler than stimulants.
Its dual function as both a laxative and antacid makes it unusual among over-the-counter options. In fact, when magnesium hydroxide is used in combination antacid products, it’s often paired with aluminum salts specifically to counteract the laxative effect, so you get the acid relief without the loose stools. If you’ve ever noticed that some antacid liquids contain both magnesium and aluminum, that’s why.