Lactulose

How it works

It is not absorbed from the gut and is fermented by colonic bacteria to organic acids, which exert an osmotic effect that increases water content and softens the stool, promoting bowel movement.

Prescribing in practice

• It should be avoided in galactosaemia and in patients with intestinal obstruction, and used with caution in lactose intolerance.
• Its laxative effect may take a couple of days to develop, so it is not suitable for rapid relief of constipation.
• Flatulence and abdominal cramps are common, particularly when starting, and tend to settle with continued use.

Monitoring

Routine monitoring is not generally required; assess the clinical response and adjust the dose to produce a comfortable, soft stool.

Counselling the patient

• The medicine may take a day or two to work, so do not increase the dose too quickly.
• Wind and mild cramping are common at first and usually settle; maintain a good fluid intake.

Evidence & guidelines

Lactulose is a long-established osmotic laxative recommended for constipation in national prescribing guidance.