Milk of magnesia actually is used as an antacid, and it’s FDA-approved for that purpose. The real issue is that the dose needed for acid relief sits uncomfortably close to the dose that sends you to the bathroom. At antacid levels (1 to 3 teaspoons), magnesium hydroxide neutralizes stomach acid effectively. But the laxative dose is only 2 to 4 tablespoons, meaning there’s a narrow margin between settling your stomach and triggering diarrhea. This practical limitation is the main reason most people reach for other antacids instead.
The Laxative Problem
Magnesium is poorly absorbed in the gut. When it lingers in the intestinal lumen, it pulls water in through osmosis, increasing the fluid content of your stool. At low doses, this effect is mild enough to ignore. But magnesium hydroxide’s acid-neutralizing power and its laxative power come from the same molecule, so taking a bit more than intended, or taking it repeatedly throughout the day for persistent heartburn, can easily tip you into osmotic diarrhea.
Compare that to calcium carbonate (the active ingredient in Tums and Rolaids), which tends to cause the opposite problem: constipation. For someone dealing with occasional heartburn who just wants relief without side effects, calcium carbonate is a simpler choice. You chew a tablet, your symptoms ease, and your bowels stay predictable. With milk of magnesia, the tradeoff is less forgiving.
How It Compares to Other Antacids
Magnesium hydroxide reacts rapidly with stomach acid to produce magnesium chloride and water. In clinical comparisons, a combination antacid containing magnesium hydroxide and aluminum hydroxide started working faster than calcium carbonate alone. That combination also lasted longer in the esophagus (about 82 minutes versus 60 minutes for calcium carbonate) and raised stomach pH for around 26 minutes, while calcium carbonate didn’t significantly raise stomach pH compared to placebo in the same study.
So magnesium hydroxide is, if anything, a more effective acid neutralizer. The reason it lost the popularity contest isn’t about efficacy. It’s about format and convenience. Calcium carbonate comes as a chewable tablet you can carry in a pocket or purse. Milk of magnesia is a thick, chalky liquid that requires measuring. For on-demand heartburn relief, most people want something portable and easy, not a bottle and a measuring spoon.
This is partly why magnesium hydroxide shows up more often in combination antacids, paired with aluminum hydroxide. Aluminum compounds cause constipation while magnesium compounds cause loose stools, so combining them balances out the gut side effects. Many pharmacy-brand liquid antacids use this formula. You’re still getting magnesium hydroxide as an antacid in those products, just not on its own.
The Kidney Safety Factor
Your kidneys clear excess magnesium from the bloodstream efficiently, so occasional use at antacid doses is safe for most people. The concern arises with repeated use or impaired kidney function. When the kidneys can’t filter magnesium fast enough, blood levels rise, a condition called hypermagnesemia. Kidney failure is the most common cause, but people with even moderate kidney disease who regularly take magnesium-containing products can develop it.
Symptoms of excess magnesium range from nausea and muscle weakness to dangerously low blood pressure and irregular heartbeat in severe cases. This risk doesn’t apply to a single dose for heartburn, but it does make doctors cautious about recommending milk of magnesia as a go-to antacid for chronic acid issues. Calcium carbonate and acid-reducing medications like H2 blockers don’t carry this particular risk, making them easier to recommend broadly.
Why Doctors Steer People Away
Gastroenterologists tend to discourage regular milk of magnesia use even as a laxative, preferring gentler osmotic options for long-term constipation management. The same caution extends to its antacid use. The American College of Gastroenterology notes that antacids are used “exclusively for on-demand symptom relief” and finds little evidence favoring one type over another for occasional heartburn. During pregnancy, magnesium-containing antacids are considered safe as first-line options alongside calcium and aluminum-based products.
The clinical reality is that anyone with heartburn frequent enough to need daily antacids should probably be on something stronger, like an H2 blocker or a proton pump inhibitor, rather than any antacid at all. For the occasional bout of acid reflux where an antacid makes sense, calcium carbonate is simply more practical: it’s a tablet, it works, and it won’t loosen your stool.
When Milk of Magnesia Makes Sense
If you’re someone who deals with both occasional heartburn and a tendency toward constipation, milk of magnesia at an antacid dose (1 to 3 teaspoons) could actually work in your favor. The mild stool-softening effect that makes it a poor choice for most heartburn sufferers becomes an advantage if constipation is already part of your picture.
It also remains a perfectly effective option if you don’t mind the liquid format and you’re careful with dosing. The key is staying at the lower end. Three teaspoons (15 mL) is the maximum antacid dose. The laxative dose starts at 30 mL, so doubling up because your heartburn persists means you’ve crossed into laxative territory. If a single antacid dose doesn’t resolve your symptoms, switching to a different class of medication makes more sense than taking more.