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Osmotic laxatives

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Osmotic laxatives act by retaining fluid in the bowel by an osmotic effect. They act mainly in the small bowel however, lactulose also lowers colonic pH thereby stimulating the large bowel.

Osmotic laxatives include:

  • lactulose, lactitol - lactulose (and lactitol) only should be used when other laxatives have failed to have an effect on the constipation (1). These agents are very sweet and may be unpalatable to some patients. They commonly cause flatulence, abdominal cramps and bloating. These agents must be regularly taken for up to 3 days before an effect is seen. These agents are therefore unsuitable for rapid relief of constipation
  • macrogols (Movicol, Idrolax) are the most recently introduced laxatives
    • side effects of macrogols include abdominal distension, pain and nausea
    • a review on laxatives stated "..Although macrogols have shown some benefits over placebo and lactulose in small, short-term trials, there is insufficient evidence to recommend their routine use over other well-established, less expensive agents.."
  • magnesium salts - these result in rapid bowel evacuation. If they are given in large doses then defaecation may occur in one to two hours. These laxatives should be reserved for bowel clearance prior to surgical procedures. Apart from in patients with megarectum, they are not suitable for regular use
  • enemas - phosphates (rectal); sodium citrate (rectal)


  1. MeReC Bulletin 1999; 10 (9): 33-36.
  2. MeReC Bulletin 2004;14(6):21-4.

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