Osmotic Laxatives for Constipation

Can osmotic laxatives help you find relief?

Osmotic laxatives are a type of laxative (stool softener) used in the management of constipation. Osmotic laxatives come in over-the-counter and prescription forms, and they work by increasing the flow of water into the intestines to produce softer easier-to-pass stools.

If you have chronic constipation, you should see your doctor so that you can get a diagnosis and a treatment plan. Medications are often part of the treatment plan in managing chronic constipation.

laxatives for constipation
Illustration by Jessica Olah, Verywell

How Osmotic Laxatives Work

Normally, solid waste and water pass through your intestines, and eventually to your colon. Constipation occurs when the waste cannot easily pass along your colon, often resulting in straining, infrequent small stools, and discomfort.

Water normally flows between the lumen (tube) of the colon where the stool passes and the blood vessels in the wall of the colon. The amount of water that flows in either direction is largely based on maintaining a balance between the concentration of water in your colon and the concentration of water outside your colon. Factors such as the content of the waste material in your colon, the food you ate, and the medications you use can influence how much water is mixed with your stool. That's where osmotic laxatives come in.

Adding water to the solid waste in the colon can soften it so that it passes more easily.

Osmotic laxatives are small particles or sugars that promote the flow of water into the lumen of the colon through the process of osmosis, which is the passive flow of water that balances the concentration of fluid and solids. The result of using osmotic laxatives is softer, more watery stools.

Common Osmotic Laxatives

There are several common osmotic laxatives you can use if you have constipation.


MiraLAX and GlycoLax (polyethylene glycol PEG) are over-the-counter medications that increase the content of molecules in the colon, drawing water into the stool to reduce constipation.


Lactulose is a prescription medication. It is a sugar that is not absorbed by the digestive system, and this makes it available to be fermented (metabolized by microbes in the intestines). This fermentation process produces fatty acids that promote the flow of water into the colon and increase the speed of intestinal contractions.

Lactulose is sold under a variety of brand names, including Cephulac, Cholac, Chronulac, Constilac, Constulose, Duphalac, Enulose, Generlac, and Kristalose.


Sorbitol is a natural sugar. When used as an osmotic laxative, it is taken to promote the flow of water into the intestines. Sorbitol is available in an over-the-counter form and a prescription form.

Magnesium Citrate

A salt, over-the-counter magnesium citrate draws water into the lumen of the colon, softening the stool.

Milk of Magnesia

Considered a saline laxative, Milk of Magnesia is available over the counter. This product is rarely recommended by physicians today because safer and more effective alternatives exist.

You should avoid Milk of Magnesia if you have heart disease or kidney disease.

The Side Effects of Osmotic Laxatives

Osmotic laxatives produce a variety of side effects. The most common side effects include symptoms of nausea, bloating, cramping, flatulence, and diarrhea.

Overusing these products or using them when you have another underlying health issue can result in life-threatening side effects, including dehydration and an electrolyte imbalance.

Dehydration: Losing too much fluid through your stool can make your body deficient in water. This causes dry skin, fatigue, a rapid heart rate, and may even cause you to pass out.

Electrolyte imbalance: Excessive diarrhea can alter the balance of your body's electrolytes —like sodium, potassium, can calcium. This can cause serious heart rhythm irregularities, seizures, and organ failure.

A Note From Verywell

If you are considering the use of an osmotic laxative, be sure to follow dosing instructions carefully. To avoid side effects and complications, use a laxative only when needed and try to limit their long-term use. If you have recurrent constipation, discuss your symptoms with your doctor, and don't rely on chronic use of laxatives to relieve your symptoms.

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Article Sources

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  2. Tropini C, Moss EL, Merrill BD Transient Osmotic Perturbation Causes Long-Term Alteration to the Gut Microbiota. Cell. 2018 Jun 14;173(7):1742-1754.e17. doi: 10.1016/j.cell.2018.05.008.