Medically reviewed by Zara Risoldi Cochrane, Pharm.D., M.S., FASCP — Written by Amanda Barrell on November 21, 2018

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Milk of magnesia is a standard, effective treatment for constipation. People can buy it from drug stores without a prescription.

Share on PinterestMilk of magnesia can relieve constipation and ease heartburn and indigestion.
Milk of magnesia contains magnesium, which is a naturally occurring mineral. The human body requires magnesium to help many of its systems function properly, most notably the muscles and the nerves.

Milk of magnesia is also known as magnesium hydroxide, which is its chemical name. Milk of magnesia is available to buy over the counter without a prescription.

People should not give milk of magnesia to children under 2 years of age unless advised by a doctor.

People use milk of magnesia as a laxative to relieve constipation, and to ease indigestion and heartburn. This is because it can reduce the amount of stomach acid while increasing water in the intestines.

Magnesium hydroxide works by drawing water from the tissue around the bowel into the gut to ‘flush’ the contents out.

The original form of milk of magnesia usually produces a bowel movement in 30 minutes to 6 hours.

Milk of magnesia is one of the most commonly used over-the-counter treatments for constipation. Constipation is when a person is passing stools fewer than three times a week.

People experiencing constipation may report the following symptoms:

hard, lumpy stoolsbloatingnot feeling fully empty after going to the toiletdiscomfort in the abdominal areafeeling the need to strain excessively

Milk of magnesia is available to buy as either a tablet or a liquid. When using the tablet form, a person usually needs to chew tablet before swallowing.

Milk of magnesia is available as a regular strength liquid or a concentrated liquid. People should not give the concentrated liquid to children under the age of 12.

People can buy different forms of milk of magnesia from drug stores or online.


People should not take more medication than is recommended on the package.

To take liquid milk of magnesia, a person can mix it with milk or water. Shake the bottle well before measuring out a dose. The dosage varies depending on why the person is using the medicine.

Milk of magnesia for constipation

People of all ages should drink a full glass, or 8 ounces, of water with each dose of milk of magnesia. Use the 15 ml dosing cup or spoon provided for accuracy. It is best to take the medication at bedtime.

Using the original version of milk of magnesia for constipation, the dosage in milliliters (ml) varies depending on a person’s age:

adults can take 30 – 60 mlchildren aged 6 to 11 can take 15 – 30 mlask a doctor before giving this medication to children under 6 years old

For the concentrated version of milk of magnesia, the dosage is lower:

adults can take 15 – 30 mlask a doctor before giving this medication to children under 12 years old

There are also chewable tablets for children. Children should drink a full glass of liquid with each dose. The dosage varies depending on age:

children aged 6 to 13 can take 3–6 tablets per daychildren aged 2 to 6 can take 1–3 tablets per dayask a doctor before giving this medication to children under 2 years old

People should not take milk of magnesia as a laxative for more than 7 days in a row. Anyone who is still in need of a laxative or has persistent pain in their stomach area should talk to a doctor.

Milk of magnesia usually relieves constipation within 6 hours of taking it. If a person does not have a bowel movement after using milk of magnesia, they should stop using it and talk to a doctor. There may be another cause of constipation that doctors can treat.

Milk of magnesia for other digestive issues

Along with constipation relief, people can also use some versions of milk of magnesia to relieve heartburn and acid indigestion.

Adults should take 5 – 15 ml at a time with water, and repeat up to 4 times per day as needed. They should not take more than 60 ml in any 24-hour period.

When using milk of magnesia as an antacid, it may also have a laxative effect. Do not use milk of magnesia as an antacid for more than 14 days in a row.

Talk to a doctor before using milk of magnesia to treat other digestive issues in children under 12.

Most people who take milk of magnesia do not experience side effects.

The most common side effects of milk of magnesia are:

stomach crampschalky tastefeeling nauseousvomiting

Milk of magnesia also carries more serious side effects. People who experience any of the following should stop using the medicine straight away and seek medical attention:

rectal bleedingno bowel movement after taking itsevere nausea or vomitingslow heartbeatlight-headedness

Serious side effects are more likely to occur if a person takes more milk of magnesia than is recommended, or if they take it for an extended period.

People who are taking this medicine need to make sure they drink plenty of water to prevent becoming dehydrated. If anyone experiences diarrhea after taking a dose of milk of magnesia, they should avoid taking it again.

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If someone overdoses on milk of magnesia, they should seek emergency medical attention. Symptoms of an overdose may include:

severe diarrheamuscle weaknessmood changeslow or irregular heartbeatlittle or no urination

Some people may be allergic to milk of magnesia. Signs of an allergic reaction that require medical attention include:

difficulty breathingswelling of the face, lips, tongue, or throat

People with impaired kidney function should avoid milk of magnesia.