Osmotic Laxatives

Osmotic laxatives are poorly absorbed substances that retain water in the bowel lumen and soften stool. Lactulose is also fermented by colonic bacteria to organic acids that acidify the colon and trap ammonia, which makes it the basic treatment for hepatic encephalopathy.

Pharmacokinetics. Minimal absorption. Onset is 24 to 48 hours.

• Lactulose 15 mL (10 g) twice daily initially, adjusted to a soft daily stool.
• Lactitol 10 g once daily.
• Macrogol 3350 with electrolytes, 1 to 3 sachets daily; for faecal impaction 8 sachets daily dissolved in 1 litre of water, drunk over 6 hours, for up to 3 days.
• Magnesium hydroxide and magnesium sulphate.
• Rectal phosphate or sodium citrate enemas.
• Oral sodium phosphate bowel preparations.

Differences. Macrogol is best tolerated and most effective. Lactulose causes more bloating and flatulence. Phosphate and magnesium products carry electrolyte risk.

• Chronic and opioid-induced constipation: macrogol or lactulose, with a stimulant added when needed.
• Hepatic encephalopathy: lactulose 30 to 45 mL (20 to 30 g) three to four times daily, titrated to 2 to 3 soft stools daily; by enema 300 mL in 700 mL of water for patients who cannot swallow.
• Faecal impaction: high-dose macrogol after rectal measures.
• Bowel preparation: macrogol 4 litres in a split-dose regimen.

Contraindications. Bowel obstruction, ileus, toxic megacolon, galactosaemia (lactulose). Phosphate products in kidney impairment, heart failure, ascites, dehydration, older adults and children under 2 years.

Adverse effects. Bloating, cramps, flatulence, diarrhoea and dehydration. Magnesium salts: hypermagnesaemia in kidney impairment. Phosphate: hyperphosphataemia, hypocalcaemia and acute phosphate nephropathy.

Interactions. Hypokalaemia with diuretics and corticosteroids raises digoxin toxicity. Lactulose should not be given with other laxatives in hepatic encephalopathy before the response is measured.

Special groups.

• Pregnancy: lactulose and macrogol acceptable.
• Breastfeeding: acceptable.
• Children: use paediatric macrogol preparations with product dosing.
• Older adults: monitor fluids and electrolytes.
• Kidney impairment: avoid magnesium and phosphate.

Monitoring. Stool number, sodium, potassium, magnesium, creatinine and hydration. In encephalopathy, mental state.

Referral and escalation. Persistent constipation after combination treatment, or encephalopathy not responding to lactulose within 48 hours, needs specialist review and a search for precipitants.