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What are Osmotic Laxatives?

Medically Reviewed by May Zhu, RDN | Published March 28, 2024

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Osmotic laxatives are commonly used medications to relieve constipation by drawing water into the bowel, softening stool, and promoting bowel movements. In this blog, we’ll review the mechanics of osmotic laxatives and explore different categories this laxative category, along with examples. 

How Do Osmotic Laxatives work? 

Osmotic laxatives work by drawing water into the colon, which softens stool and promotes bowel movements. They are commonly used to alleviate constipation and are available in various forms, including oral solutions, powders, and tablets.

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Examples of Osmotic Laxatives

Polyethylene glycol (PEG):PEG is a commonly prescribed osmotic laxative available in powder form. It is tasteless, odorless, and typically mixed with water or other fluids before consumption. PEG works by increasing the water content in the colon, softening stool, and facilitating bowel movements.

Lactulose: Lactulose is a synthetic sugar solution that acts as an osmotic laxative by drawing water into the colon and promoting the growth of beneficial bacteria. It is often prescribed to treat chronic constipation and hepatic encephalopathy.

Magnesium hydroxide: Magnesium hydroxide, also known as milk of magnesia, is an osmotic laxative that works by attracting water into the intestines, leading to increased bowel movements. It is commonly used to relieve occasional constipation and is available in liquid and tablet forms.

Comparison Chart of Osmotic Laxatives

Osmotic Laxative

Mechanism of Action


Common Uses

Polyethylene Glycol

Draws water into the colon


Relieving constipation, bowel preparation


Attracts water, promotes bacterial growth


Chronic constipation, hepatic encephalopathy

Magnesium Hydroxide

Attracts water into intestines

Liquid, tablets

Occasional constipation relief


Osmotic laxatives, such as polyethylene glycol, lactulose, and magnesium hydroxide, work by drawing water into the colon to soften stool and promote bowel movements. 

View Citation

  • [1] Bharucha AE, Pemberton JH, Locke GR. American Gastroenterological Association technical review on constipation. Gastroenterology. 2013;144(1):218-238.
  • [2] Wald A. Constipation: advances in diagnosis and treatment. JAMA. 2016;315(2):185-191.
  • [3] National Institute for Health and Care Excellence (NICE). Constipation in children and young people: diagnosis and management. 2017. Available from: