lactulose
osmotic laxative· Brands: Duphalac, Generlac, LaxosePOPR
Mechanism of action
Lactulose is a synthetic disaccharide. It is not absorbed from the gastrointestinal tract and reaches the colon unchanged. There, colonic bacteria metabolise it to organic acids such as lactic acid and acetic acid. These acids increase intraluminal osmotic pressure, drawing water into the colon, softening stools and increasing faecal volume, which stimulates peristalsis and promotes defecation. The resulting acidic environment also reduces the absorption of ammonia from the colon, an effect that underpins its use in hepatic encephalopathy.
Pharmacokinetics
Lactulose is poorly absorbed from the small intestine, with less than 3% of an oral dose reaching the systemic circulation. The unabsorbed fraction is metabolised in the colon by resident bacteria. The onset of laxative action is usually 24 to 48 hours after administration, though it can take up to 72 hours. For hepatic encephalopathy, improvement is typically seen within 24 to 48 hours. Any small amount that is absorbed is excreted unchanged in the urine.
Indications & dosing
Constipation
Usually given as a single dose in the morning. Response may take up to 48 hours.
- adult · PO: 15–30 mL Once daily · Usually given as a single dose in the morning. Response may take up to 48 hours.
Hepatic encephalopathy (portal-systemic encephalopathy)
Dose adjusted to produce 2-3 soft stools per day. May also be given as 200 g in 1 L of water per rectum.
- adult · PO: 30–45 mL 3-4 times daily · Dose adjusted to produce 2-3 soft stools per day. May also be given as 200 g in 1 L of water per rectum.
Contraindications & precautions
- Galactosaemia · Lactulose is a synthetic disaccharide containing galactose; it is contraindicated in patients with galactosaemia.
- Intestinal obstruction · May exacerbate obstruction and cause electrolyte disturbances; avoid in suspected obstruction.
- Perforation or risk of perforation · Avoid in patients with known or suspected gastrointestinal perforation or toxic megacolon.
- Severe dehydration or electrolyte disturbances · Lactulose-induced diarrhoea may worsen fluid and electrolyte imbalance; use with caution and monitor electrolytes.
- Use with caution in diabetes mellitus; lactulose contains galactose and may affect blood glucose control. Monitor blood glucose in diabetic patients.
- Monitor serum electrolytes and fluid balance during prolonged therapy, especially with high doses used for hepatic encephalopathy.
- In patients with a colostomy or ileostomy, be aware of altered faecal output and fluid balance; take care to prevent dehydration.
- Use with caution in renal impairment, as electrolyte disturbances may be more likely.
Adverse effects
- Flatulence
- Bloating
- Abdominal cramps
- Diarrhoea
- Electrolyte disturbancesserious
- Dehydrationserious
- Aspirationserious
Monitoring, pregnancy & toxicity
- Bowel frequency and stool consistency · Daily in hepatic encephalopathy; as needed for constipation
- Serum electrolytes (sodium, potassium) · Periodically during long-term use or in patients at risk of electrolyte imbalance
- Neurological status (consciousness, asterixis) · Regularly in hepatic encephalopathy
- Pregnancy: Lactulose is generally considered safe during pregnancy. It acts locally in the colon and is poorly absorbed, resulting in minimal systemic exposure. No teratogenic effects have been reported. Use when clinically indicated.
- Toxicity: Profuse watery diarrhoea · Stop lactulose; monitor fluid balance and electrolytes; provide supportive care with oral or intravenous fluids and electrolyte replacement.
- Toxicity: Electrolyte disturbances (e.g., hypokalaemia, hypernatraemia) · Monitor electrolytes and correct imbalances; reduce or discontinue lactulose.
- Toxicity: Dehydration · Encourage oral fluids if able; administer intravenous fluids if severe.
Clinical notes
Lactulose is used for the treatment of chronic constipation and for the management of hepatic encephalopathy. For constipation in adults, a typical starting dose is 15 to 30 ml of the syrup (containing approximately 10 to 20 g of lactulose) once daily, which can be increased if necessary. For hepatic encephalopathy, higher doses are often used, starting with 30 to 45 ml three to four times daily, then adjusted to produce two to three loose stools daily. Lactulose is contraindicated in patients with galactosaemia and should be used with caution in patients with diabetes mellitus because of its sugar content. Common side effects include flatulence, abdominal cramps and diarrhoea, especially at higher doses. Prolonged use may lead to electrolyte disturbances. It is available as a syrup and as an enema (for rectal administration).
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BNF; local formulary.