The standard adult dose of Milk of Magnesia for constipation is 2 to 4 tablespoons (30 to 60 mL), taken once in a 24-hour period. The exact amount depends on whether you’re using it as a laxative or an antacid, your age, and whether you have the original or concentrated formula.
Adult Laxative Dose
For constipation relief, adults and children 12 and older take 30 mL (2 tablespoons) to 60 mL (4 tablespoons) of the original-strength liquid. Start at the lower end and increase only if needed. Do not exceed 60 mL in a 24-hour period. Drink a full glass of water with your dose to help it work properly and prevent dehydration.
Most people take it at bedtime. The laxative effect kicks in anywhere from 30 minutes to six hours later, so a bedtime dose typically produces a bowel movement by morning. If you’re sensitive to it, you may want to take it earlier in the evening so you’re not woken up in the middle of the night.
Original vs. Concentrated Formula
This is where dosing mistakes happen most often. The concentrated version of Milk of Magnesia contains 2,400 mg of magnesium hydroxide per 10 mL, roughly double the strength of the original formula. That means you need about half the volume to get the same effect. Always check the label on your specific bottle before measuring a dose, because pouring 4 tablespoons of the concentrated version would put you well over the safe limit.
Doses for Children
Children under 2 should not take Milk of Magnesia unless directed by a pediatrician. For older children, the dose scales down by age. Children ages 6 to 11 typically take about half the adult dose (15 mL, or 1 tablespoon), and children ages 2 to 5 take roughly a quarter of the adult dose (5 to 15 mL). These ranges vary by product, so read the packaging carefully. The same rule applies to kids: give a full glass of liquid alongside the dose.
Using It as an Antacid
When you’re taking Milk of Magnesia for heartburn or acid indigestion rather than constipation, the dose is significantly smaller. Antacid doses are typically 1 to 3 teaspoons (5 to 15 mL) and can be repeated up to a few times per day, depending on the product label. The key difference is frequency: laxative dosing is one large dose per day, while antacid dosing is smaller amounts taken as symptoms arise. Stick with the directions on your specific product, since formulations differ.
How Long You Can Safely Use It
Milk of Magnesia is meant for short-term, occasional use. Most labels recommend no more than 7 consecutive days as a laxative. If you still need it after a week, that’s a signal something else is going on with your digestion that deserves attention. Long-term use can disrupt your body’s electrolyte balance, particularly magnesium levels, and your bowels can become dependent on the stimulation.
People with kidney disease face a specific risk. Healthy kidneys filter out excess magnesium efficiently, but impaired kidneys cannot. Magnesium can build up to dangerous levels in the blood, causing muscle weakness, low blood pressure, and in severe cases, heart rhythm problems. If you have any degree of kidney disease or are on a magnesium-restricted diet, this is not a grab-it-off-the-shelf product for you.
Timing Around Other Medications
Magnesium hydroxide interacts with a long list of medications. There are over 400 known drug interactions, including common ones like thyroid medication (levothyroxine), blood pressure drugs, antibiotics, and even daily vitamins. The magnesium can bind to other drugs in your stomach and reduce how much your body absorbs. The general approach is to separate Milk of Magnesia from other medications by at least 2 hours, but check with your pharmacist if you take anything daily. This is especially important for thyroid hormones and antibiotics, where reduced absorption can make the medication ineffective.
Tips for Getting the Dose Right
Shake the bottle well before pouring. Milk of Magnesia is a suspension, meaning the active ingredient settles to the bottom. If you pour without shaking, your first dose may be mostly water and your last dose may be a concentrated sludge.
Use the measuring cup that comes with the product, not a kitchen spoon. A “tablespoon” from your silverware drawer can vary widely in actual volume. For accuracy, measure in milliliters. If the liquid is too chalky or unpleasant, flavored versions (mint and cherry are common) are available at the same strength. You can also follow it with a few sips of juice to wash the taste away.
If nothing happens within 6 hours of taking a dose, do not take a second one. Wait until the next day and try again at the higher end of the dosage range. Taking extra to “speed things up” increases your risk of cramping, diarrhea, and electrolyte imbalance without making the laxative work faster.