Milk of magnesia typically produces a bowel movement within 30 minutes to 6 hours after you take it. The active ingredient, magnesium hydroxide, has an elimination half-life of about 8 hours, meaning its effects generally wind down within a day. But “how long does it last” can mean several things, so let’s cover all of them.
How Quickly It Works
Most people notice results somewhere in the 30-minute to 6-hour window. The wide range exists because speed depends on what’s already in your digestive system, how much water you drink with the dose, and how backed up things are. Taking it on an empty stomach with a full glass of water tends to produce faster results. If nothing happens after 6 hours, that’s worth noting, but it doesn’t necessarily mean the dose failed entirely.
How Long the Effect Lasts in Your Body
Magnesium hydroxide works as an osmotic laxative, meaning it draws water into your intestines. That extra fluid softens stool and triggers the muscles in your bowel to contract. Once this process starts, it typically runs its course within several hours.
The elimination half-life of magnesium hydroxide is roughly 8.3 hours. In practical terms, most of the drug clears your system within about a day. Around 80% of the dose passes through your digestive tract unabsorbed and leaves in your stool. The remaining portion (up to about 20%) is absorbed into the bloodstream and filtered out through your kidneys. So while the laxative effect is concentrated in that first 6-hour window, trace amounts of magnesium continue circulating for hours afterward.
The common side effects during this window include bloating, abdominal discomfort, and cramping. These typically fade once the bowel movement passes.
Dosage Limits and Duration of Use
For adults using the liquid form as a laxative, the maximum recommended dose is 60 mL (about 4 tablespoons) in a 24-hour period. You should not use milk of magnesia for more than one week without talking to a doctor. It’s considered a first-line treatment for uncomplicated constipation, but it’s meant as a short-term fix, not a daily habit.
If you find yourself reaching for it regularly, that’s a signal to look at fiber intake, hydration, and other underlying causes rather than continuing to rely on a laxative.
How Long It Lasts as an Antacid
Milk of magnesia also works as an antacid at lower doses, neutralizing stomach acid to relieve heartburn and indigestion. In this role, it acts faster (usually within minutes) but doesn’t last as long as some other antacids. The acid-neutralizing effect is relatively short-lived because the liquid moves through your stomach fairly quickly. You can expect antacid relief to last roughly 1 to 3 hours, depending on whether you’ve eaten recently. Food in the stomach slows things down and can extend the effect slightly.
Shelf Life of the Bottle
An unopened bottle of milk of magnesia stays effective until the expiration date printed on the packaging. Store it at room temperature (68 to 77°F) and avoid freezing, which can alter its consistency and effectiveness. Don’t use it if the lid seal is open or damaged.
Once opened, milk of magnesia doesn’t spoil the way food does, but it can slowly lose potency over time. The expiration date on the package remains your best guide. If the liquid looks discolored, has an unusual smell, or has separated in a way that shaking doesn’t fix, it’s time to replace it.
Who Should Avoid It
Milk of magnesia is safe for most adults when used as directed, but it’s not appropriate for everyone. People with kidney problems should be especially cautious. Because the kidneys are responsible for clearing the absorbed magnesium, impaired kidney function can cause magnesium to build up in the bloodstream. This can lead to dangerously high magnesium levels, which affect heart rhythm and muscle function. Kidney failure is a direct contraindication, and even mild kidney insufficiency calls for extra caution.
People taking prescription medications should also check for interactions, since magnesium hydroxide can interfere with how certain drugs are absorbed. Taking other medications at least 2 hours before or after your dose of milk of magnesia helps avoid this problem.