osmotic laxatives for constipation

An osmotic laxative is a stool softener used to treat or prevent constipation. Available over the counter or by prescription, they work by increasing the flow of water into the intestines.

This article explains how osmotic laxatives work and how they differ from other types of laxatives. It also lists some of the more common osmotic laxatives along with possible risks and side effects.

How osmotic laxatives work

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

The term “osmosis” refers to the movement of fluid through a membrane such that the concentrations on both sides are equal. This is how osmotic laxatives work.

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

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

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


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

How Osmotic Laxatives Are Different

Osmotic laxatives work differently than other types of laxatives because they are sometimes used to prevent or treat chronic constipation. Others are generally used to treat 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 promote the diffusion of fats and water. Docusate increases the passage of water and fat into stools, making them softer.
  • Lubricant laxatives: These are made with oily substances, such as mineral oil, to make it easier for stool to pass through the intestines.
  • Stimulant laxatives: These are a type of laxative that relieve constipation by causing the intestines to contract and pass stool.


Osmotic laxatives work differently than emollient laxatives (which draw water and fat into the stool), lubricating laxatives (which lubricate the stool), and stimulant laxatives (which speed up bowel contractions).

Common osmotic laxatives

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

  • Polyethylene glycol (PEG): This is an organic compound derived from petroleum that can be safely ingested to control constipation. PEG-containing laxatives available over the counter include MiraLAX and GlycoLax.
  • Lactulose: This is a sugar that is not absorbed by the gut. Instead, sugars are fermented in the gut, producing fatty acids that draw water into the gut lumen. Available by prescription, lactulose-containing laxatives include Cephulac, Duphalac, Kristalose, and others.
  • Sorbitol: This is another non-absorbable sugar that acts like 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. Salt helps draw water into the lumen. OTC versions include Citrate of Magnesia, Citroma and LiquiPrep.
  • Magnesium Hydroxide: This is a milder form of magnesium that is traded under the name Milk of Magnesia. Available over the counter, milk of magnesia can also be used as an antacid.


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

possible side effects

Like all medicines, osmotic laxatives can cause side effects. Most are relatively mild and resolve on their own within a few days.

Common side effects of osmotic laxatives include:

  • nausea
  • bloating
  • abdominal cramps
  • flatulence
  • diarrhea

Overuse of osmotic laxatives can lead to dehydration and loss of electrolytes such as sodium, calcium, and potassium. These are some of the minerals your body needs to regulate your heartbeat, muscle contractions, and other key functions.

Although not approved for this class, osmotic laxatives such as Miralax are sometimes used for the long-term management of chronic constipation. Other osmotic laxatives are often used for short-term use.

Talk to your healthcare provider to make sure you are using any laxatives, whether over-the-counter or prescription, correctly.


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

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Osmotic laxatives can help treat or prevent constipation by drawing water into the colon. This action helps soften stools and make them easier to pass. Some osmotic laxatives can be used in bowel preparation to help clear stool from the colon before 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 hydroxide. Some (like Miralax and Milk of Magnesia) are available over the counter, while others (like Cephulac and Kristalose) are only available by prescription.

Osmotic laxatives can cause side effects such as nausea, bloating, cramping, gas, and diarrhea. Overuse of osmotic laxatives can lead to dehydration and other potentially serious complications.

VigorTip words

If you are considering an osmotic laxative, be sure to follow the dosage instructions carefully. To avoid complications, use laxatives only when needed.

If you have chronic constipation, talk to your healthcare provider so they can determine the underlying cause. In some cases, the condition can improve with diet, exercise, and increased fluid intake. Others may require treatment beyond the use of laxatives.