Osmotic Laxatives for Constipation

Osmotic laxatives are a type of 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 and easier-to-pass stools.

If you have chronic constipation, you should see your doctor to determine if testing is required to evaluate you for conditions that can present as stooling irregularity, such as thyroid and celiac diseases. Your doctor can provide a treatment plan that includes medications, as well as dietary and lifestyle changes, to help manage your constipation.

How Osmotic Laxatives Work

Normally, solid material passes through your small intestines during digestion, and whatever is left eventually finds its way into your colon (large intestine). Constipation occurs when the waste cannot easily pass along your colon, often resulting in straining, infrequent small stools, and discomfort.

Water is free to move between the lumen, or tube of the colon, where the stool passes the blood vessels in the wall of the colon. The net direction in which water moves in the colon is determined by the relative concentrations of water in the lumen and in the wall of the colon. When you're well-hydrated, fluid can more easily move from the wall of the colon and into the lumen, thereby softening any stool that's present. In contrast, when you're dehydrated, the wall of your colon may be inclined to absorb water from the lumen of the colon, thereby making any stool present there harder and more difficult to pass. 

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. Additional factors that can impact the consistency of your stool include the food you've eaten and any medications you're taking. That's where osmotic laxatives come in.

Common Osmotic Laxatives

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

MiraLAX

MiraLAX and GlycoLax (polyethylene glycol PEG) are over-the-counter medications that increase the content of non-digestible and non-absorbable molecules in the colon. This results in the movement of water into the intestinal lumen and softens any stool that's present.

Lactulose

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

Sorbitol is a natural and non-absorbable sugar. It is used as a sweetener in sugar-free gum and some fruit juices. 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

Magnesium citrate draws water into the lumen of the colon, thereby softening the stool.

Milk of Magnesia

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, and 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.