Laxatives, osmotic

Key examples

• Lactulose
• Macrogol
• Phosphate enema

 

Common indications

1. Constipation and faecal impaction.
2. Bowel preparation prior to surgery or endoscopy.
3. Hepatic encephalopathy.

 

Mechanisms of action

• These medicines are based on osmotically active substances (sugars or alcohols) that are not digested or absorbed, and which therefore remain in the gut lumen. They hold water in the stool, maintaining its volume and stimulating peristalsis. Lactulose, in particular, also reduces ammonia absorption. It does this by increasing gut transit rate and acidifying the stool, which inhibits the proliferation of ammonia-producing bacteria. This is helpful in patients with liver failure, in whom ammonia plays a major role in the pathogenesis of hepatic encephalopathy.

 

Important adverse effects

• Flatulence, abdominal cramps and nausea are common adverse effects, although they may decrease with time. As with other laxatives, diarrhoea is a possible complication. Phosphate enemas can cause local irritation and electrolyte disturbances.

 

Warnings

• Osmotic laxatives are contraindicated in intestinal obstruction as there is a risk of perforation. 
• Phosphate enemas can cause significant fluid shifts so should be used with caution in heart failure, ascites and when electrolyte disturbances are present.

 

Important interactions

• There are no significant adverse drug interactions with osmotic laxatives, although the effects of warfarin may be slightly increased.