Milk of magnesia (magnesium hydroxide) is a short-term, over-the-counter constipation treatment. It’s best to take the dose listed on the label at bedtime with a full glass of water, making sure to wait 2 hours before or after taking other medications.

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.

During constipation, stool remains in the bowel longer, becoming harder and drier. This makes it more difficult to pass.

Milk of magnesia is often effective for short-term constipation relief, but it’s not ideal for treating chronic constipation.

A common cause of mild or temporary constipation is a low fiber diet. Choosing foods like fruits, vegetables, and whole grains can increase the fiber in your diet.

Drinking too little water can have the same negative effect. Staying hydrated is important for many reasons, including healthy bowels.

A sedentary lifestyle can also reduce the frequency of your bowel movements.

More serious health issues can also cause constipation. Irritable bowel syndrome, thyroid disease, an anal blockage or tumor, and diabetes are among the conditions that can lead to constipation.

Some other factors that may cause constipation include:

  • ignoring the urge to have bowel movements
  • certain medications like narcotics, iron pills, or blood pressure-lowering drugs
  • traveling
  • pregnancy
  • some pelvic floor disorders
  • a change in medication
  • a change in diet

Milk of magnesia is a type of saline 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.

These kinds of laxatives are also known as “salts.” They’re meant to be fast-acting. You can expect to have a bowel movement anywhere from 30 minutes to 6 hours after 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 is not an appropriate option.

Milk of magnesia comes in both tablet and liquid form.

Whichever form of the medication you choose, read the label carefully to determine the proper amount for children, as it’s different than the proper amount for adults. If anything on the label confuses you, it’s always a good idea to reach out to a medical professional to have them walk you through it.

It’s recommended to take either the tablet or the liquid with a full, 8-ounce glass of water.

Generally, it’s advised to take milk of magnesia at bedtime.

If you’re taking any other medications, try to take those either 2 hours before, or 2 hours after, taking milk of magnesia.

Most people over age 6 can safely take milk of magnesia. For children under 6 years old, parents should speak with their child’s doctor first.

Check with your doctor before taking milk of magnesia if you:

  • are on a magnesium-restricted diet
  • take any prescription drugs, since some can interact with milk of magnesia
  • are pregnant or breastfeeding, in which case you should talk 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.

The main side effect of taking milk of magnesia, or any laxative, is diarrhea.

If diarrhea occurs or you become nauseated, stop taking milk of magnesia. If a more serious side effect results, like rectal bleeding, see your doctor right away. This could indicate a more serious health concern.

It’s important to note that extended use or overuse of this medication for constipation may result in dependence on laxatives and ongoing bowel issues.

There are three main lifestyle choices to consider 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
  • whole grain bread and cereals

Drink plenty of fluids

Staying well-hydrated is an important part of preventing constipation. While the “8 glasses a day” saying does not have much clinical evidence to support it, it’s a good idea to drink water often. By the time you feel thirsty, you’re likely already slightly dehydrated.

Get moving

A lack of physical activity, as well as living with obesity, can contribute to constipation. Exercise is important and health-promoting. Decisions like getting up often during the workday, taking the stairs when possible, and even parking farther away from entrances to stores can be beneficial as well.

Milk of magnesia is the common name for magnesium hydroxide, which is a type of saline laxative. It’s recommended for occasional constipation. You can expect to have a bowel movement within 6 hours of taking milk of magnesia. Sometimes bowel movements can occur in as little as half an hour.

The nature and cause of your constipation may affect 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 create an effective treatment that works well with other medications you take.