Yes, Milk of Magnesia is a laxative. Its active ingredient, magnesium hydroxide, is classified as an osmotic laxative, meaning it works by drawing water into the colon to soften stool and trigger a bowel movement. It typically produces results within 30 minutes to six hours of taking a dose. While it’s best known for relieving constipation, the same ingredient also works as an antacid at lower doses, which is why you’ll sometimes see it marketed for heartburn and acid indigestion as well.
How It Works as a Laxative
Osmotic laxatives like Milk of Magnesia pull water from surrounding tissues into your intestines. This extra fluid does two things: it softens the stool so it’s easier to pass, and the increased volume stretches the intestinal walls enough to stimulate the muscles that push things along. The effect is relatively fast compared to other types of laxatives, with many people noticing results in under an hour.
How It Differs From Other Laxatives
Not all laxatives work the same way, and the differences matter when you’re choosing one.
- Osmotic laxatives (Milk of Magnesia) draw water into the colon, softening stool and stimulating the bowels.
- Stimulant laxatives (senna, bisacodyl) act directly on the intestinal walls, signaling the nerves and muscles to contract and push stool through. These tend to cause more cramping.
- Stool softeners (docusate sodium) simply increase the water content of existing stool, making it softer. They don’t actively trigger a bowel movement, so they’re gentler but slower and less reliable for acute constipation.
Milk of Magnesia sits in the middle ground: more effective than a stool softener, but generally less harsh than a stimulant laxative.
Dosage for Constipation
The standard laxative doses, based on FDA labeling, are straightforward. Adults and children 12 and older can take 30 mL (2 tablespoons) to 60 mL (4 tablespoons). Children ages 6 to 11 take 15 mL (1 tablespoon) to 30 mL (2 tablespoons). Children under 6 should not take it without a doctor’s guidance. The maximum for anyone in a 24-hour period is 60 mL.
When used as an antacid rather than a laxative, the dose is much smaller, typically 10 to 20 mL between meals or at bedtime, and shouldn’t exceed 60 mL per day or be continued for more than two weeks.
How Quickly It Works
Most people can expect a bowel movement somewhere between 30 minutes and six hours after taking a dose. The wide range depends on factors like how much you took, how hydrated you are, and what’s in your stomach. Taking it on an empty stomach with a full glass of water tends to speed things up. If you haven’t had a bowel movement after several hours, that’s a sign something else may be going on.
Common and Serious Side Effects
The most frequent side effects are diarrhea, stomach cramps, nausea, and vomiting. These are usually mild and resolve on their own. Because the medication pulls water into your intestines, it can leave you mildly dehydrated, so drinking plenty of water alongside it is important.
More serious reactions are rare but worth knowing about. Rectal bleeding, no bowel movement after taking a dose, severe nausea or vomiting, a slow heartbeat, or lightheadedness all warrant stopping the medication and getting medical attention. If you experience diarrhea after your first dose, you shouldn’t take it again.
Who Should Avoid It
People with kidney disease should not use Milk of Magnesia. Healthy kidneys clear excess magnesium from the bloodstream without trouble, but impaired kidneys can’t keep up. Magnesium accumulates, potentially reaching dangerous levels that affect heart rhythm and muscle function. Kidney disease guidelines specifically list magnesium-containing laxatives as products to avoid entirely.
Medications That Don’t Mix Well
Magnesium hydroxide can interfere with how your body absorbs several common medications, including certain antibiotics (ciprofloxacin, tetracycline), digoxin, iron supplements, and thyroid hormones like levothyroxine. If you take any of these, spacing them at least two hours apart from your Milk of Magnesia dose helps prevent the interaction from reducing your medication’s effectiveness.