February 23, 2023/Digestive

Laxative Effect: Using Milk of Magnesia for Constipation

Developed in the 1800s, this remedy remains a viable solution today

Person suffering from constipation with someone handing them medicine in a small cup.

Glass bottles lining pharmacy shelves in the 1800s offered “fluid magnesia” options for those experiencing constipation and tummy troubles. The problem? The taste of these elixirs was not the best.


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So, Charles H. Phillips decided to develop a more palatable option. His creation — dubbed “Milk of Magnesia” and patented in 1873 ­— can still be bought today.

But is a constipation remedy that nearly dates to the U.S. Civil War a good choice given advances in pharmaceutical products and medical knowledge? Let’s find out from gastroenterologist Samita Garg, MD.

What is milk of magnesia?

As you might have guessed, “milk of magnesia” isn’t a dairy-based item. The name is a catchy marketing moniker that captures the liquid product’s milky white appearance and use of magnesium.

The technical name of “milk of magnesia” is magnesium hydroxide, or Mg(OH)2. It’s essentially a laxative.

How does milk of magnesia help constipation?

“Constipation” is the medical term for when you just don’t poop enough. As your bowel movements become less frequent, poop lingers in your large intestine (colon) and typically becomes dry and hard.

Those rocks of stool become increasingly hard to pass, leading to a plugged-up feeling that often gets folks reaching for various over-the-counter remedies and natural solutions for constipation relief.

Which brings us to milk of magnesia. The product works to resolve constipation in two ways.

  1. It draws water from your body to your intestines. (“It’s what we call an osmotic effect,” explains Dr. Garg.) This increase in moisture softens poop that may be getting a little too big and solid.
  2. It stimulates nerves in your intestines to help push poop through your system. This leads to peristalsis, or a wavelike constriction and relaxation of muscles that pushes poop through your system.

Or to put the process in even simpler terms: Milk of magnesia unclogs you.


How fast does milk of magnesia work?

Put it this way: Don’t stray too far from a restroom. Milk of magnesia can get things moving in as little as 30 minutes, notes Dr. Garg.

In general, the expected timeframe for a bowel movement is anywhere from 30 minutes to six hours after taking milk of magnesia. (It’s often recommended to take milk of magnesia before bedtime.)

Tips for taking milk of magnesia

Milk of magnesia is available in liquid, pill or chewable tablet form. Follow the directions on the package label for recommended dosage and age of use. (Instructions typically advise talking to a doctor before giving the product to children younger than age 6.)

Milk of magnesia usually is taken once a day. (But doses also may be divided. Just be sure not to exceed the recommended dosage within a 24-hour period.)

It’s best to follow a dose of milk of magnesia with an 8-ounce glass of water.

One more important note: Don’t take milk of magnesia for more than a week without talking to a doctor. “If it’s not helping with constipation or your symptoms get worse, it’s best to get evaluated to see if there’s something else going on,” states Dr. Garg.

Possible side effects

Milk of magnesia is known for being pretty gentle, but some people do experience side effects. These can include:

Stop taking milk of magnesia and contact a doctor if you see blood in your stool, experience severe vomiting or are unable to have a bowel movement even after taking the laxative.


Precautions with milk of magnesia

It’s best to talk to your healthcare provider before using milk of magnesia if you’re:

  • Pregnant or breastfeeding.
  • On a magnesium-restricted diet.
  • Being treated for kidney disease.
  • Taking prescription medications.
  • Dealing with a sudden change of bowel habits lasting more than two weeks.
  • Experiencing stomach pain, nausea or vomiting.

Other uses for milk of magnesia

Aside from constipation relief, milk of magnesia also is used as an antacid to relieve heartburn, indigestion and upset stomach. (Learn more about ways you can tame heartburn in our Health Essentials Podcast.)

Bottom line

There’s a reason why milk of magnesia remains on store shelves 150 years after being introduced: “It has proven to be an effective way to treat mild to moderate constipation,” reaffirms Dr. Garg.

Just make sure to view milk of magnesia as a short-term treatment. If you continue to have issues with constipation, talk to a healthcare provider to find the reason why and possible solutions.

“I’m a big advocate of holistic therapies when it comes to constipation,” says Dr. Garg. “While there are supplements and over-the-counter medications that work, many times, we can address the issues causing constipation through simple lifestyle and diet changes.”

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