Milk of magnesia is among the most common over-the-counter treatments for constipation. This liquid laxative is a compound called magnesium hydroxide. It’s often effective for short-term constipation relief, but it’s not ideal for treating chronic constipation.


Constipation is a condition that affects nearly everyone at some point. It occurs when having a bowel movement is difficult or when bowel movements occur infrequently. Because the stool remains in the bowel for a longer time, it becomes hard and dry. This makes it more difficult to pass. The Mayo Clinic describes constipation as having fewer than three bowel movements per week.

What causes constipation?

A common cause of mild or temporary constipation is a low-fiber diet. Choose foods such as fruits, vegetables, and whole grains to increase the fiber in your diet. Eating too many dairy products may also cause some people to become constipated. Drinking too little water can have the same effect. Staying hydrated is important for many reasons, including healthy bowels.

A sedentary lifestyle can also reduce the frequency of your bowel movements. If you’re pregnant, you also have an increased chance of constipation.

Some medications, such as sedatives, iron pills, or blood pressure-lowering drugs, may also cause constipation.

More serious health issues can also cause constipation. Irritable bowel syndrome, thyroid disease, and colon cancer are among the conditions that lead to constipation. People with multiple sclerosis or Parkinson’s disease can sometimes experience periods of constipation.

How does milk of magnesia work?

Milk of magnesia is a type of hyperosmotic laxative. This kind of oral laxative works by drawing water to the bowel from nearby tissue. This softens and moistens the stool. It also helps increase bowel activity.

Saline, lactulose, and polymer laxatives are the three types of hyperosmotic laxatives. Milk of magnesia is a saline laxative. These kinds of laxatives are also known as “salts.” They’re meant to be fast-acting. You should expect to have a bowel movement within six hours of taking milk of magnesia.

Lactulose laxatives draw more water to the bowel from surrounding tissue, but they act more slowly than saline types. People use lactulose types for chronic constipation. If you have recurring bouts of constipation or if you need a long-term treatment, milk of magnesia isn’t an appropriate option.

Who can use milk of magnesia and who shouldn’t?

Most people over the age of 6 can safely take milk of magnesia. For children under 6, you should speak with your child’s doctor first.

You should check with your doctor before taking milk of magnesia if:

  • you have kidney disease
  • you’re on a magnesium-restricted diet
  • you take any prescription drugs, since some can interact with milk of magnesia
  • you’re pregnant or breast-feeding, in which case you should check with your doctor before taking any type of laxative

Milk of magnesia is a short-term treatment. If you need to take it often to have a bowel movement, or if you try it and you’re still not having regular bowel movements, speak with your doctor. You may have an underlying medical condition.


Read the label carefully to determine an age-appropriate amount. For example, children ages 6 to 11 can have 1 to 2 tablespoons of Phillips' Milk of Magnesia. Anyone age 12 or older can have 2 to 4 tablespoons per dose. You should have no more than one dose per day.

You should also drink an 8-ounce glass of water or other liquid with each dose.

Many supermarkets and drug stores sell milk of magnesia and other laxatives. See your doctor if you still need a laxative after a week of daily treatments. You should also talk to your doctor if your constipation is accompanied by nausea and vomiting.

Possible side effects

The main side effect of taking milk of magnesia or any laxative is diarrhea. Usually, if you take the dose recommended on the label, the result should be a normal bowel movement. Everyone reacts to medications a little differently, though. Even an appropriate dose may lead to loose stools, but that should be a temporary side effect.

If diarrhea occurs or you become nauseated, stop taking milk of magnesia. If a more serious side effect results, such as rectal bleeding, see your doctor promptly since this could indicate a more serious health concern.


Milk of magnesia should work the first time you take it. You can expect to have a bowel movement within six hours. Sometimes, this can occur in as little as half an hour. The nature and cause of your constipation may impact how long it takes for the treatment to work.

If you don’t have a bowel movement within a day or two of taking milk of magnesia, you may need a stronger treatment. If you have an underlying medical condition that may be causing constipation, be sure to discuss laxative use with your doctor. They may need to coordinate an effective treatment with other medications you take.

How to prevent constipation

There are three main lifestyle choices you can make to help prevent constipation:

Eat a high-fiber diet

Eating a high-fiber diet generally helps keep you regular. Recommended foods include berries and other fruits, green, leafy vegetables, and whole-grain bread and cereals. Reduce your intake of dairy products if you think they might be causing you digestion problems. You can take steps to get enough calcium from non-dairy sources.

Drink plenty of fluids

Staying well-hydrated is an important part of preventing constipation. Unless otherwise directed by your doctor, try to drink about eight glasses of water per day. Other types of fluids may be okay, including tea and juice. Keep in mind that juices are usually high in sugar. Beverages that contain a lot of caffeine or alcohol can act as diuretics and reduce fluid levels in the body.

Get moving

A lack of physical activity, as well as being overweight or obese, can contribute to constipation. Aim to get at least 30 minutes of moderate to intense exercise per day. You may want to try jogging, brisk walking, or aerobics. You can also consider team sports or swimming.