For occasional constipation, most adults use 400 to 800 mg of supplemental magnesium, though the effective dose depends on which form you take. Magnesium citrate, magnesium oxide, and magnesium hydroxide (sold as Milk of Magnesia) are the three forms most commonly used as laxatives, and each has its own dosing range and speed of action.
How Magnesium Relieves Constipation
Magnesium works as an osmotic laxative. The magnesium ions pull water from surrounding tissues into your colon, which softens and loosens stool so it moves through more easily. This is a mechanical effect, not a chemical stimulant, so it’s generally gentler than stimulant laxatives. You can typically expect a bowel movement within 30 minutes to 6 hours after taking a dose, though the exact timing varies from person to person.
Dosages by Form
Magnesium Citrate
Magnesium citrate is the most commonly recommended form for acute constipation. It comes as a liquid (usually in 10-ounce bottles) and as tablets. The standard adult dose is 195 to 300 mL of the liquid, taken as a single dose or split throughout the day. If you’re using tablets, 2 to 4 tablets at bedtime is typical. Drink a full glass of water with either form to help the osmotic effect work properly.
Magnesium citrate has slightly higher bioavailability than other forms, meaning your body absorbs more of it. That’s useful for raising magnesium levels overall, but it also means citrate tends to cause more gastrointestinal side effects. In a randomized trial published in the Journal of the American Heart Association, participants taking magnesium citrate reported significantly more GI symptoms than those taking magnesium oxide, and some discontinued use because of digestive complaints.
Magnesium Hydroxide (Milk of Magnesia)
Milk of Magnesia is the most familiar over-the-counter option. Adults and children 12 and older take 30 mL (2 tablespoons) to 60 mL (4 tablespoons) per day, either as a single dose (preferably at bedtime) or divided into smaller doses. Do not exceed 60 mL in 24 hours. Always drink a full 8-ounce glass of liquid with each dose.
For children ages 6 to 11, the dose drops to 15 mL (1 tablespoon) to 30 mL (2 tablespoons). Children under 6 should not take Milk of Magnesia without specific guidance from a pediatrician.
Magnesium Oxide
Magnesium oxide is widely used for constipation and is available in capsule and tablet form, typically in 250 mg or 400 mg doses. It can be taken one to four times daily depending on the product. Like the other forms, take it with a full glass of cold water or juice, and avoid taking it late in the day on an empty stomach.
Oxide has lower bioavailability than citrate, which means less magnesium gets absorbed into your bloodstream and more stays in the gut doing the osmotic work. This actually makes it effective as a laxative while causing fewer side effects. In clinical trials, no participants in magnesium oxide groups dropped out due to GI complaints, compared with about 4% of those taking citrate.
Choosing the Right Form
If you need fast, one-time relief, magnesium citrate liquid tends to work quickly and is easy to dose. If you want something gentler for repeated use over a few days, magnesium oxide or Milk of Magnesia may be better tolerated. For children, Milk of Magnesia is the most practical choice because weight-based and age-based dosing guidelines are well established: children ages 2 to 5 typically take 5 to 15 mL per day, and children 6 to 11 take 15 to 30 mL per day.
Regardless of form, magnesium laxatives are meant for short-term use. MedlinePlus advises not using magnesium oxide as a laxative for more than one week unless directed by a doctor. The same general principle applies to citrate and hydroxide.
Safety Limits and Side Effects
The tolerable upper intake level for supplemental magnesium is 350 mg per day for adults, according to the NIH. This limit applies only to magnesium from supplements and medications, not from food. Laxative doses frequently exceed this threshold, which is why they’re intended for short-term use only.
The most common side effects at laxative doses are diarrhea, nausea, and abdominal cramping. These are signs the magnesium is pulling more water into the colon than your body needs. If you experience watery diarrhea, reduce your dose next time.
Very high doses carry more serious risks. Magnesium toxicity can cause dangerously low blood pressure, vomiting, muscle weakness, difficulty breathing, and irregular heartbeat. This is rare at normal laxative doses in healthy adults, but the risk rises significantly for people with kidney disease. Impaired kidneys can’t clear excess magnesium efficiently, so it accumulates in the blood. The Mayo Clinic lists magnesium-containing products among medications that are typically not safe for people with kidney disease.
Dosing for Children
Pediatric dosing of magnesium hydroxide is based on both age and weight. The weight-based guideline is 80 to 240 mg per kilogram of body weight per day, divided into one or two doses. For a 20-kilogram (44-pound) child, that works out to roughly 1,600 to 4,800 mg per day, though most children need far less than the upper end of that range.
The practical age-based ranges for Milk of Magnesia (using the standard 400 mg per 5 mL concentration) are:
- Ages 2 to 5: 5 to 15 mL per day (400 to 1,200 mg)
- Ages 6 to 11: 15 to 30 mL per day (1,200 to 2,400 mg)
- Ages 12 and older: 15 to 60 mL per day (1,200 to 4,800 mg)
Overdosing in children can cause elevated magnesium levels in the blood along with drops in phosphate and calcium, so start with the lower end of the range and increase only if needed.
Tips for Getting the Dose Right
Start at the low end of any dosing range. You can always take more, but you can’t undo a dose that sends you to the bathroom for hours. Take magnesium with plenty of water, both to activate the osmotic effect and to prevent dehydration from the fluid shift into your colon. If you’re using the liquid form, drinking it cold makes the taste more tolerable.
Give it the full 6 hours before deciding it hasn’t worked. Many people take a second dose too soon and end up with cramping and diarrhea. If a full dose of magnesium citrate or Milk of Magnesia produces no result after 6 hours, the constipation may need a different approach rather than a higher magnesium dose.