Osmotic Laxatives for Constipation

Mature man taking medicine
BSIP/UIG/Universal Images Group/Getty Images

Osmotic laxatives are used to treat constipation. Although the products that are classified as osmotic laxatives are varied, in general they all work by increasing the amount of water that is secreted within the intestines. This effect helps to produce softer, easier-to-pass stools. Some osmotic laxatives are available over the counter while others require a prescription. Here is an overview of the three most common types of osmotic laxatives:


Miralax (polyethylene glycol PEG) is a medication that draws water into the stool, resulting in a softer stool and inducing more frequent bowel movements. No need for a prescription, Miralax is available over the counter. There is one published ​report that Miralax was more effective than Zelnorm in increasing the number of bowel movements and improving symptoms in patients who suffer from chronic constipation. Miralax seems to cause fewer gastrointestinal side effects (such as bloating and flatulence) than the other osmotic laxatives.


Lactulose is not absorbed by the digestive system and thus is available to be metabolized by microbes in the intestines. This fermentation process produces fatty acids that pull 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.

Milk of Magnesia

Considered a saline laxative, Milk of Magnesia is available over the counter. This product is rarely recommended by physicians in the modern era because safer and more effective alternatives exist. Milk of Magnesia should be avoided by anyone who suffers from heart or kidney disease.

    Side Effects of Osmotic Laxatives

    Side effects of osmotic laxatives vary by the type of product used. In general, the side effects of osmotic laxatives are fairly minimal. When side effects are present, these are the symptoms reported:

    In rare cases, some of the side effects of osmotic laxatives can be severe and life-threatening. These are most often experienced by people who overuse the products or who have serious, underlying health issues.

    • dehydration
    • electrolyte imbalance

      Do They Work?

      The most recent review of quality research on osmotic laxatives conducted by The American College of Gastroenterology (ACG) finds good support for the effectiveness of Lactulose and Miralax in "improving stool frequency and consistency" in patients who suffer from chronic constipation. A previous review concluded that there was no substantial evidence for the effectiveness of Milk of Magnesia.

        The Bottom Line

        Osmotic laxatives, particularly Lactulose and Miralax, appear to be safe, effective options for easing constipation.

        If you are considering the use of an osmotic laxative, discuss this with your physician​ and be sure to follow dosing instructions carefully.

        For more laxative options:


          American College of Gastroenterology Chronic Constipation Task Force "An Evidence-Based Approach to the Management of Chronic Constipation in North America" The American Journal of Gastroenterology 2005 100:S1-S4.

          Cash, C., Chang, L., Sabesin, S. & Vitat, P. "Update on the Management of Adults With Chronic Idiopathic Constipation" The Journal of Family Practice 2007 S13-S20.

          Di Palma, J. " A Randomized, Multicenter Comparison of Polyethylene Glycol Laxative and Tegaserod in Treatment of Patients With Chronic Constipation" American Journal of Gastroenterology 2007 102:1964-1971.

          Ford, A., et.al. "American College of Gastroenterology Monograph on the Management of Irritable Bowel Syndrome and Chronic Idiopathic Constipation" American Journal of Gastroenterology 2014 109:S2-S26.

          Lembo, A. & Camilleri, M. "Chronic Constipation" The New England Journal of Medicine 2003 349:1360-1368.

          Continue Reading