Osmotic Laxatives for Constipation

How They Work and Differ From Other Laxatives

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Osmotic laxatives are medications used to treat or prevent constipation. They draw extra water into your stool, making it softer and easier to pass. Osmotic laxatives are available over the counter and by prescription. Examples include Milk of Magnesia, lactulose, and polyethylene glycol (PEG).

This article explains how osmotic laxatives work, how they differ from other types of laxatives, plus the possible risks and side effects.

laxatives for constipation
Illustration by Jessica Olah, Verywell

How Osmotic Laxatives Work

Constipation occurs when stools are infrequent and hard to pass. The stools will usually be hard and dry. Osmotic laxatives can help relieve constipation by increasing the amount of fluid in the intestines. This, in turn, softens stools and makes them easier to pass.

The term "osmotic" refers to the movement of a fluid through a membrane so that the concentration is equal on both sides. This is how osmotic laxatives work.

In people with constipation, the concentration of water in the wall of the colon and the inside of the colon (called the lumen) will be balanced but too low to compensate for hard, dry stools. This is especially true of people who are don't consume enough water.

Osmotic laxatives alter the balance with substances—such as salts, sugars, and other organic compounds—that encourage the movement of water into the lumen.

In addition to treating constipation, osmotic laxatives are sometimes used for bowel prep (to cleanse the bowel of stool) prior to undergoing colonoscopy.


Osmotic laxatives work by drawing water from the wall of the colon to the inside of the colon. This helps soften stools and makes them easier to pass.

How Osmotic Laxatives Differ

Osmotic laxatives work differently than other types of laxatives in that they are sometimes be used to prevent or treat chronic constipation. The others are generally used for the treatment of occasional constipation.

Other types of laxatives include:

  • Emollient laxatives: These are a type of laxative made with a surfactant called docusate. Surfactants are substances that encourage the spread of fats and water. Docusate increases the passage of water and fats into stools to make them softer.
  • Lubricant laxatives: These are made with oily substances, like mineral oil, that make it easier for stool to slip through the intestine.
  • Stimulant laxatives: These are a type of laxative that relieves constipation by causing the intestines to contract and push out stools.


Osmotic laxatives work differently than emollient laxatives (that draw water and fat to stools), lubricant laxatives (that lubricate stools), and stimulant laxatives (that speed intestinal contractions).

Common Osmotic Laxatives

There are several common osmotic laxatives you can use if you have constipation. Each is made with different active ingredients:

  • Polyethylene glycol (PEG): This is an organic compound derived from petroleum that can be safely ingested to manage constipation. Available over the counter, PEG-containing laxatives include Miralax and GlycoLax.
  • Lactulose: This is a type of sugar that is not absorbed by the intestine. Instead, the sugar sits and ferments in the intestines, producing fatty acids that draw water into the lumen. Available by prescription, lactulose-containing laxatives include Cephulac, Duphalac, Kristalose, and many others.
  • Sorbitol: This is another non-absorbable sugar with an action similar to lactulose. Over-the-counter (OTC) and prescription versions are available, including Arlex and GeriCare.
  • Magnesium citrate: Magnesium citrate is magnesium in salt form combined with citric acid. The salts help draw water into the lumen. OTC versions include Citrate of Magnesia, Citroma, and LiquiPrep.
  • Magnesium hydroxide: This is a milder form of magnesium sold under the brand name Milk of Magnesia. Available over the counter, Milk of Magnesia is also used as an antacid.


There are several different active ingredients used in osmotic laxatives, including polyethylene glycol (Miralax), lactulose (Cephulac), Sorbitol (Arlex), magnesium citrate (Citrate of Magnesia), and magnesium hydroxide (Milk of Magnesia).

Possible Side Effects

As with all drugs, osmotic laxatives can cause side effects. Most are relatively mild and will resolve on their own within a couple of days.

Common side effects of osmotic laxatives include:

  • Nausea
  • Bloating
  • Abdominal cramping
  • Flatulence
  • Diarrhea

The overuse of osmotic laxatives can cause dehydration and the loss of electrolytes like sodium, calcium, and potassium. These are some of the minerals that the body needs to regulate heartbeats, muscle contractions, and other key functions.

Although not approved for such, osmotic laxatives like Miralax are sometimes used for the long-term management of chronic constipation. The other osmotic laxatives are generally intended for short-term use.

Speak with your healthcare provider to ensure that you are using any laxative correctly, whether it is over-the-counter or prescription.


Common side effects of osmotic laxatives include nausea, bloating, cramping, flatulence, and diarrhea. The overuse of osmotic laxatives can lead to dehydration and other complications.


Osmotic laxatives can help treat or prevent constipation by drawing water into the colon. This action helps soften stools and makes them easier to pass. Some osmotic laxatives can be used for bowel preparation to help clear the colon of stool in advance of a colonoscopy.

There are different types of osmotic laxatives that contain different active ingredients. These include polyethylene glycol (PEG), lactulose, sorbitol, magnesium citrate, and magnesium hydroxides. Some (like Miralax and Milk of Magnesia) are available over the counter, while others (like Cephulac and Kristalose) are available by prescription only.

Osmotic laxatives can cause side effects like nausea, bloating, cramping, gas, and diarrhea. The overuse of osmotic laxatives can cause dehydration and other potentially serious complications.

A Word From Verywell

If you are thinking about using osmotic laxatives, be sure to follow the dosing instructions carefully. To avoid complications, use a laxative only when needed.

If you have chronic constipation, see your healthcare provider so that they can identify the underlying cause. In some cases, the condition can be improved with diet, exercise, and increased fluid intake. Others may require treatments that extend beyond the use of laxatives.

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Tropini C, Moss EL, Merrill BD. Transient Osmotic Perturbation Causes Long-Term Alteration to the Gut Microbiota. Cell. 2018;173(7):1742-1754.e17. doi:10.1016/j.cell.2018.05.008

By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.