What is lactulose, and how does it work (mechanism of action)?

Lactulose is a man-made sugar that contains two naturally occurring sugars, galactose and fructose. It is not digested in the intestine like other sugars so that it reaches the colon where bacteria digest it and thereby alter the composition of the stool.

Lactulose is used as a laxative to treat constipation. In the colon, lactulose is broken down by bacteria into products that help to draw water into the colon, which softens the stool.

Additionally, lactulose is used to treat hepatic encephalopathy, a loss of brain function and change in mentation that occurs when the liver is unable to remove toxins from the blood. Bacteria in the colon digest lactulose into chemicals that bind ammonia that is believed to be the toxin that causes hepatic encephalopathy. The binding of ammonia prevents ammonia from moving from the colon into the blood and also draws ammonia from the blood and into the colon. The bound ammonia then is removed from the body in the stool.

The FDA approved lactulose in March, 1976.

What brand names are available for lactulose?

Constulose, Enulose, Generlac, Cholac, Constilac

Is lactulose available as a generic drug?


Do I need a prescription for lactulose?


What are the side effects of lactulose?


Diarrhea (loose stool) may occur if the dose of lactulose is too high. Problems associated with diarrhea are fluid and potassium loss in the diarrheal stool leading to dehydration and low blood levels of potassium (hypokalemia). An additional side effect is the elevation of blood levels of sodium (hypernatremia) as a result of the loss of fluid.

Lactulose contains sugars (galactose and lactose) and should be used cautiously in people with diabetes; however, since lactulose is not digested, and little of the sugar is absorbed, the effects in people with diabetes usually are minimal.

What is the dosage for lactulose?

The usual dose is 1 to 2 tablespoonfuls (15 to 30 mL, containing 10 g to 20 g of lactulose) daily. The dose may be increased to 60 mL daily if necessary. Twenty-four to 48 hours may be required to produce a normal bowel movement.

Note: Some patients have found that lactulose solution may be more acceptable when mixed with fruit juice, water or milk.

Dosages from FDA prescribing information for lactulose

Is lactulose safe to take if I’m pregnant or breastfeeding?

It is not known whether lactulose is secreted in human milk or cause harm to the nursing infant.

What else should I know about lactulose?

What preparations of lactulose-oral are available?
  • Oral or rectal solution: 10 mg/15 ml
  • Powder for solution (single dose packets): 10 and 20 g.
How should I keep lactulose-oral stored?

Lactulose products should be stored at room temperature, between 15 C and 30 C 59 F and 86 F).

Medically Reviewed on 3/5/2019


Medically reviewed by John P. Cunha, DO, FACOEP; Board Certified Emergency Medicine


FDA Prescribing Information