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Milk of magnesia is an over-the-counter (OTC) treatment for constipation. It works by drawing water into the bowel and softening stool, making it easier to pass. It can also soothe indigestion and heartburn.

This article explains what milk of magnesia is, how to use it, what conditions it can treat, common side effects, and more.

Milk of magnesia, also known as magnesium hydroxide, can act as an antacid or as a saline laxative.

This type of laxative works by drawing moisture into a person’s bowels to help loosen stool.

A person may purchase milk of magnesia over the counter (OTC) without a prescription, but they should talk with their doctor if they experience frequent bouts of constipation.

Parents and guardians should avoid giving milk of magnesia to children under 6 years old unless their doctor recommends its use.

People use milk of magnesia for:

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

In addition to constipation and acid relief, milk of magnesia may help with conditions such as sunburn and acne, though empirical evidence is significantly lacking.

Milk of magnesia for sunburn

Some people believe that applying a thin layer of milk of magnesia topically to sunburn will help ease the pain and burning sensation.

Though this may work for some people, no studies or empirical evidence exist to support its use on sunburn.

A person interested in sunburn relief should talk with their doctor. Other OTC products, such as aloe vera, may work better for a person.

Milk of magnesia for acne

Some people believe that milk of magnesia can help with treating acne. The belief stems from the medication’s potential ability to help break up surface oils.

The only research on its use for acne dates back to a 1975 study. In the study, a researcher noted that the use of milk of magnesia combined with orally taking 250 milligrams (mg) of tetracycline and washing two times daily with a nonfat soap helped clear up acne pustules and reduce inflammation.

No further studies have looked at the use of applying milk of magnesia topically or taking it orally for the treatment of acne. A person should talk with their doctor about alternative methods to treat acne.

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 the 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 their doctor or the packaging recommends.

Though dosing can vary, a person should avoid taking more than the recommended dose over the course of 24 hours.

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 medication and their age.

The following sections describe the doses for milk of magnesia based on use and age.

Milk of magnesia for constipation

People who are old enough to take milk of magnesia should drink a full glass, or 8 ounces, of water with each dose of milk of magnesia. Use the 15-milliliter (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 varies depending on a person’s age:

  • Adults can take 30–60 ml.
  • Children ages 6–11 can take 15–30 ml.
  • Ask 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 ml.
  • Ask 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 ages 6–12 can take 3–6 tablets per day.
  • Children ages 2–6 can take 1–3 tablets per day.
  • Ask 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 with 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 with a doctor.

In those cases, an underlying condition may be the cause of constipation and may need additional treatment from a doctor.

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 four 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.

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

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

The most common side effects of milk of magnesia are:

  • diarrhea
  • stomach cramps
  • nausea
  • vomiting
  • skin flushing
  • drowsiness

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

  • rectal bleeding
  • no bowel movement after taking it
  • severe nausea or vomiting
  • slow heartbeat
  • lightheadedness

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

People who are taking this medication 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 not use it again.

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

  • severe diarrhea
  • muscle weakness
  • mood change
  • slow or irregular heartbeat
  • little or no urination

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

  • hives
  • difficulty breathing
  • swelling of the face, lips, tongue, or throat

People living with impaired kidney function should avoid milk of magnesia. The following people should also avoid taking the medication:

  • people on a magnesium-restricted diet
  • people with symptoms such as nausea, diarrhea, or stomach pain
  • people who experience sudden bowel changes that last longer than 14 days

Magnesium may be able to cross the placenta into the body of the fetus. However, doctors do not know if milk of magnesia is safe for use during pregnancy, as there is no data on this.

Small amounts of magnesium may also make its way into breast milk, but doctors do not know the safety of this either.

As such, the general advice is to avoid using milk of magnesia when pregnant or nursing or ask a doctor before use.

Milk of magnesia interferes with a wide range of medications, which means it affects how they work. These include prescription and OTC medications, as well as vitamins and supplements.

Because of the way it impacts the liquids in the gut, milk of magnesia can stop tablets from being absorbed properly.

As a result, a person should talk with their doctor before using milk of magnesia if they regularly take other medications, including OTC medications, vitamins, and prescription medications.

Milk of magnesia is a well-known and effective laxative for the short-term treatment of constipation.

People should not use milk of magnesia for more than 7 days at a time for constipation or 14 days at a time for other digestive issues.

Ongoing symptoms can be a sign of a more serious gut health condition, so if the problem persists, a person should contact their doctor.

Milk of magnesia works by drawing water into the bowel from the surrounding tissue. This means it can stop the body from absorbing a range of other medications, including prescription drugs, supplements, and vitamins.

Anyone who takes medication for a health condition should talk with a doctor before taking milk of magnesia.