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Laxatives are medications used to treat constipation. Based on their mechanism of action, they can be categorized as:


  • Bulk-forming laxatives

  • Stool softeners

  • Osmotic laxatives

  • Stimulant laxatives


Bulk-forming laxatives


As its name suggests, this type of laxative absorbs water and increases stool bulk. There are both natural and synthetic substances that belong to this category:


  • Psyllium - natural fiber from the husks of the Plantago Ovata seed

  • Wheat bran -  outer shell of a wheat grain

  • Methylcellulose - synthetic fiber derived from cellulose (Citrucel)

  • Calcium polycarbophil - synthetic polymer, (FiberCon)

  • Wheat dextrin - natural carbohydrate (Benefiber)


Of the above, psyllium is the laxative for the efficacy of which there is the strongest clinical evidence. Psyllium improves stool frequency with minimal side effects (gas, bloating). Expected onset of action: varies, but it may take several weeks to achieve desired results. Read more about psyllium and how to use it.

So many types of laxatives, which one is the right one for you

Stool softeners 


By mixing fat and water in stool, stool softeners lubricate stool and increase its bulk. The best known laxatives in this category are docusate sodium and docusate calcium (Colace, Doc-Q-Lace, DulcoEase). Despite their widespread use, the evidence in support of the efficacy of stool softeners is lacking. The expected onset of action: 12-72 hours.


Osmotic laxatives


Osmotic laxatives attract and retain fluid in the intestinal cavity, making stool softer and bulkier. Different types of substances are included in this group of laxatives:


  • Saline agents: magnesium hydroxide (milk of magnesia), magnesium citrate and magnesium sulfate (Epsom salt). Patients with kidney and heart problems should avoid this type of laxative. Improper use can lead to magnesium toxicity and dehydration. Expected onset of action: 1-3 hours.


  • Poorly absorbed sugars: lactulose (prescription medicine, Duphalac, Constilac, Chronulac, Cephulac) and sorbitol. Lactulose undergoes bacterial fermentation in the colon, releasing substances that not only have osmotic action but can increase intestinal motility as well. Bacterial fermentation also causes side effects such as gas, bloating and cramps. Sorbitol, a sugar alcohol present in many fruits and fruit juices, has similar efficacy to lactulose, but its side effects are milder. Expected onset of action: 1-2 days.


  • Polyethylene glycol (PEG, MiraLax, GlycoLax). PEG is an inert organic polymer that shows great efficacy in improving stool frequency and consistency with minimal side effects. Because it is fast-acting, effective and well tolerated, PEG is often preferred over other laxatives. Expected onset of action: 0.5-1 hour.


Stimulant laxatives 


Stimulant laxatives increase contractions of the intestinal muscle, which helps move waste down the colon. The most common stimulant laxatives in use today are senna (Senokot, Ex-Lax, Castoria) and bisacodyl (Dulcolax, Correctol). Abdominal cramping and discomfort are often reported as side effects. Stimulant laxatives are frequently recommended as “rescue” agents when other laxatives, such as PEG, are not effective. Expected onset of action: 0.5 -12 hours.


Contrary to the popular belief, the use of stimulant laxatives does not seem to permanently damage the colon or increase the risk of colorectal cancers.   However, overuse of stimulant laxatives has been associated with dehydration and imbalance of electrolytes (ions with important functions in the body), potassium in particular. In addition, some people, mostly females, use stimulant laxatives for weight loss. This is a dangerous practice that leads to temporary weight loss, caused not by the reduction of body fat, but by the elimination of waste and water.


New pharmacologic agents


Novel types of treatments, such as Lubiprostone (Amitiza) and Linaclotide (Linzess) are available by prescription. They are more expensive than other types of laxatives and are more commonly used for severe constipation or in cases where all other treatments have failed.


When choosing laxatives, keep their cost, effectiveness, and safety in mind.

Consider increasing dietary fiber and/or using psyllium as the first step in treating your constipation. They are inexpensive, safe and can have beneficial effects on your overall health. If fiber proves ineffective for you, choose a laxative that is proven efficient and leads to minimal side effects, such as PEG.

Last updated: November 23, 2016

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