Milk of magnesia is taken by mouth as a liquid, either as a laxative for constipation or as an antacid for heartburn and indigestion. The dose, timing, and amount of water you drink with it all depend on which purpose you’re using it for. Here’s what you need to know to take it correctly.
Shake the Bottle First
Milk of magnesia is a suspension, meaning the active ingredient settles to the bottom of the bottle over time. Always shake it well before measuring your dose. Use the measuring cup or dosing spoon that comes with the product rather than a kitchen spoon, which can be inaccurate. Drink a full glass of water (8 ounces) with each dose. The water helps the product work properly and keeps you hydrated, which is especially important when using it as a laxative.
Dosage for Constipation
For the standard concentration (400 mg per 5 mL), adults and anyone 12 or older typically take 30 to 60 mL per day. That’s roughly 2 to 4 tablespoons. You can take the full amount at bedtime as a single dose, or split it into smaller doses throughout the day. A concentrated version (800 mg per 5 mL) is also available, which cuts the volume in half: 15 to 30 mL per day.
Children ages 6 to 11 take a smaller amount: 15 mL (1 tablespoon) to 30 mL (2 tablespoons) of the regular-strength liquid. Children under 6 should not take milk of magnesia unless a doctor specifically recommends it.
Dosage for Heartburn or Indigestion
When used as an antacid, the dose is smaller and taken more frequently. The label on your specific product will list the exact amount, but antacid doses are generally lower than laxative doses because you only need enough to neutralize stomach acid, not draw water into the intestines. Children under 12 should not use it as an antacid without a doctor’s guidance.
When to Expect Results
Milk of magnesia works by pulling water into your intestines, which softens stool and triggers your bowels to move. Most people have a bowel movement within 30 minutes to 6 hours after taking a dose. Taking it at bedtime is a common strategy so it works overnight, and you have a bowel movement in the morning. If nothing happens within 6 hours, don’t take a second dose right away. Give it time.
How Long You Can Use It
Milk of magnesia is meant for short-term, occasional use. Most labels recommend no more than 7 consecutive days. If you still need it after a week, that’s a sign something else may be going on with your digestion that deserves attention. Using it regularly over weeks or months can disrupt your body’s electrolyte balance, particularly magnesium levels.
Who Should Be Careful
Your kidneys are responsible for clearing magnesium from your body. If you have any degree of kidney disease or reduced kidney function, magnesium can build up in your blood to dangerous levels. Magnesium toxicity can cause muscle weakness, low blood pressure, breathing difficulty, and in severe cases, cardiac problems. If your kidney function is compromised, you should only use milk of magnesia under medical supervision with monitoring of your magnesium levels.
You should also avoid it if you have severe abdominal pain, nausea, or vomiting, as these could indicate a bowel obstruction or another condition that a laxative would make worse. People on a magnesium-restricted diet should skip it entirely.
Timing Around Other Medications
Milk of magnesia can interfere with how your body absorbs other medications. It changes the acid level in your stomach and binds to certain drugs in your digestive tract, reducing their effectiveness. This is a concern with blood thinners, cholesterol-lowering medications, blood pressure medications, and certain antibiotics. If you take any prescription medications, the safest approach is to separate them from your milk of magnesia dose by at least 2 hours. Take your regular medications first, wait, then take the milk of magnesia.
Tips for a Better Experience
Milk of magnesia has a chalky taste that many people find unpleasant. Keeping the bottle in the refrigerator makes it easier to swallow. Some brands sell flavored versions (mint and cherry are common) that mask the taste. You can also follow your dose immediately with juice or another flavored drink.
Staying well-hydrated throughout the day helps the laxative effect work more predictably. If you take it on an otherwise empty stomach at bedtime, the results tend to be faster and more consistent than when taken alongside a heavy meal, which slows down how quickly it moves through your system.