Magnesium hydroxide works in two distinct ways depending on the dose. At lower doses, it neutralizes stomach acid on contact. At higher doses, it draws water into the intestines to soften stool and trigger a bowel movement. This dual action is why the same product, commonly sold as Milk of Magnesia, appears in both the antacid and laxative aisles.
How It Neutralizes Stomach Acid
When you take a small dose of magnesium hydroxide for heartburn or indigestion, it reacts directly with the hydrochloric acid your stomach naturally produces. The reaction converts the acid into water and dissolved magnesium ions, both of which are harmless. This raises the pH inside your stomach, reducing the burning sensation almost immediately.
Unlike some antacids that can cause a rebound effect where the stomach produces even more acid afterward, magnesium hydroxide is relatively mild. It doesn’t shut down acid production the way stronger medications do. Instead, it simply neutralizes the acid that’s already there, which is why the relief tends to be quick but temporary.
How It Relieves Constipation
At laxative doses, magnesium hydroxide uses a principle called osmosis. Magnesium is poorly absorbed in the gut, so when a large amount reaches the intestines, it creates a high concentration of dissolved particles. Water naturally flows toward that concentration, pulling fluid from surrounding tissue into the intestinal space. This extra water softens and bulks up the stool, making it easier to pass.
The increased volume of fluid in the intestines also stretches the intestinal wall, which triggers wave-like muscle contractions called peristalsis. These contractions are the same ones your body uses during normal digestion to push food along, but the extra stimulation speeds things up considerably. The combination of softer stool and stronger contractions is what produces a bowel movement, typically within 30 minutes to 6 hours after taking a dose.
How Much the Body Actually Absorbs
Most of the magnesium hydroxide you swallow stays in your digestive tract and never enters your bloodstream. That’s by design: its job is local, either in the stomach or the intestines. Some magnesium does get absorbed, primarily in the small intestine, with absorption reaching roughly 80% completion within about 6 hours. The amount absorbed increases with higher doses and when taken on an empty stomach.
For most people with healthy kidneys, this absorbed magnesium is filtered out and excreted in urine without any issue. The concern arises when the kidneys can’t keep up, which is why people with reduced kidney function need to be cautious. Their bodies may not clear the extra magnesium efficiently, allowing blood levels to climb.
Side Effects to Watch For
The most common side effects are directly related to how the drug works. Because it pulls water into the intestines, it can leave the rest of your body slightly short on fluid. Signs of this include dry mouth, unusual thirst, dizziness, and reduced urination. Drinking a full 8-ounce glass of water with each dose helps counteract this effect.
The more serious concern is getting too much magnesium in the blood, a condition called hypermagnesemia. This is rare with occasional use but becomes a real risk with prolonged or heavy use. Early symptoms include muscle weakness, low blood pressure, and confusion. In severe cases, particularly with overdose, it can cause dangerously slow breathing, irregular heart rhythm, and loss of reflexes. This is why magnesium hydroxide is meant for occasional constipation relief, not daily long-term use. Prolonged reliance on any osmotic laxative can also create a dependency where the bowel becomes sluggish without it.
Interactions With Other Medications
Magnesium hydroxide can interfere with how your body absorbs several common medications. It binds to certain drugs in the gut, forming compounds that pass through without ever being absorbed. The medications most affected include fluoroquinolone antibiotics (like ciprofloxacin), tetracycline antibiotics, bisphosphonates used for bone density, and thyroid hormone replacements like levothyroxine.
If you take any of these, the standard guidance is to separate your doses by at least two hours. Take the other medication first, wait, then take the magnesium hydroxide, or vice versa. This gives each drug time to be absorbed on its own without interference.
Antacid Use vs. Laxative Use
The difference between the two uses comes down entirely to dose. A typical antacid dose is much smaller, just enough to neutralize excess stomach acid without sending a significant amount of magnesium into the intestines. A laxative dose is several times larger, delivering enough magnesium to overwhelm the small intestine’s ability to absorb it all. The unabsorbed magnesium continues into the colon, where it does its osmotic work.
This is worth understanding because accidentally taking a laxative-strength dose when you only wanted heartburn relief will likely send you to the bathroom. Always check the label for the specific use you need, as the dosing instructions differ significantly between the two applications. For laxative use, adults and children 12 and older typically follow the product’s dosing chart and can take the dose all at once or split it throughout the day.