Magnesium hydroxide is primarily used for two things: relieving constipation and neutralizing stomach acid. You’ll find it sold under brand names like Milk of Magnesia, available as a liquid or chewable tablet at virtually any pharmacy. Beyond these well-known uses, it also shows up in some natural deodorants for odor control.
Relieving Constipation
Magnesium hydroxide works as an osmotic laxative, meaning it pulls water into your intestines. That extra fluid softens stool and triggers the muscle contractions that move things along. Most people can expect a bowel movement within 30 minutes to 6 hours of taking a dose. If nothing happens after 6 hours, that’s a signal to stop and talk to a doctor rather than taking more.
The standard adult dose for constipation comes as either a liquid suspension or chewable tablets. For chewable tablets containing 1,200 mg each, adults and children 12 and older typically take 2 to 4 chews, while children ages 6 to 11 take 1 to 2. Drinking a full 8-ounce glass of water with each dose is important because the whole mechanism depends on water being available in your gut. You can take your dose all at once or split it across the day, but don’t exceed the recommended amount in 24 hours.
This type of laxative is meant for short-term, occasional use. It’s a good option when you need predictable relief from temporary constipation, like after travel or a change in diet, but it’s not designed for daily long-term use.
Neutralizing Stomach Acid
As an antacid, magnesium hydroxide directly reacts with hydrochloric acid in your stomach, neutralizing it on contact. This makes it effective for heartburn, acid indigestion, and sour stomach. The relief tends to come quickly since the chemical reaction happens almost immediately once the magnesium hydroxide reaches your stomach.
Antacid formulations often combine magnesium hydroxide with aluminum hydroxide. The reason is practical: magnesium hydroxide on its own can loosen stools (since it’s also a laxative), while aluminum hydroxide tends to cause constipation. Pairing them balances out those side effects. When using any magnesium hydroxide product as an antacid, keep use to 1 to 2 weeks at most unless a doctor has specifically told you otherwise. Persistent heartburn lasting longer than that usually points to something that needs a different approach.
Odor Control in Deodorants
Magnesium hydroxide has found a second life in natural deodorant formulas. It works by raising the pH on your skin’s surface, creating an environment where odor-causing bacteria struggle to thrive. The key distinction here is that it doesn’t block sweat the way aluminum-based antiperspirants do. Instead, it targets the bacteria responsible for turning sweat into body odor.
Because it shifts pH rather than killing bacteria indiscriminately, magnesium hydroxide leaves beneficial skin microbes largely intact. This makes it a popular choice for people who react poorly to baking soda in natural deodorants, which can be more irritating at higher concentrations. The deodorant versions typically use magnesium in forms like magnesium hydroxide or magnesium chloride, and while individual results vary, many people find them effective for moderate daily activity.
Who Should Avoid It
The biggest safety concern with magnesium hydroxide involves kidney function. Your kidneys are responsible for clearing excess magnesium from your blood, so people with kidney disease can accumulate dangerous levels. Symptoms of too much magnesium in the body include nausea, muscle weakness, low blood pressure, and in severe cases, breathing difficulties or irregular heartbeat.
Magnesium hydroxide also interferes with how your body absorbs several types of medication. It can reduce the effectiveness of certain antibiotics (including tetracyclines and a class called quinolones), osteoporosis medications in the bisphosphonate family, and some antifungal drugs. On the flip side, it can increase absorption of ibuprofen, which could intensify that drug’s effects. If you take enteric-coated tablets for any condition, be aware that magnesium hydroxide can cause those coatings to dissolve prematurely, releasing the medication in the wrong part of your digestive tract. A good rule of thumb is to separate magnesium hydroxide from other medications by at least 2 hours.
Blood in your stool after taking magnesium hydroxide is a red flag that warrants immediate medical attention. The same goes for severe cramping or no results after 6 hours, both of which suggest something beyond simple constipation may be going on.