Osmotic laxatives contain one of the following:

• Lactulose
• Macrogols (polyethylene glycols)
• Glycerol
• Magnesium hydroxide
• Magnesium sulfate.

How do they act?

Osmotic laxatives are either inorganic salts or organic compounds that are very poorly absorbed from the intestine. Their presence in the intestine creates a hypertonic state. In order to equalise osmotic pressure, water is drawn from the intestinal wall into the lumen, raising the intraluminal pressure by increasing the volume of the contents, stimulating peristalsis and promoting evacuation.

• Inorganic salts (magnesium hydroxide and sulfate). Effects are rapid; large doses produce a semi-fluid or watery evacuation within three hours and smaller doses act in six to eight hours. Magnesium salts are also believed to act by stimulating secretion of the hormone cholecystokinin, which promotes fluid secretion and motility in the intestine.
• Lactulose, a synthetic disaccharide, takes much longer to act than the inorganic osmotic laxatives because it has first to be broken down by colonic bacteria, mainly to lactic acid, which produces a local osmotic effect. It may take 72 hours of regular dosing to produce an effect, which is seen as a disadvantage by patients seeking rapid results. It has a sweet taste which makes it more palatable for children, to whom it can be given safely, but many adults find the large dose volumes required (up to 30ml) sickly and a deterrent to compliance.
• Macrogols are condensation polymers of ethylene oxide and water. Two macrogols (with molecular weights of approximately 3350 and 4000) are used as osmotic laxatives. They appear to act more effectively and rapidly than lactulose, and have been suggested as the laxative of first choice for constipation in children. Macrogols are presented as powders to be dissolved in water and are taken as a single daily dose.
• Glycerol is a highly hygroscopic trihydric alcohol that appears to exert its principal laxative action by attracting water of hydration into the intestine. It is also believed to have a direct mild irritant effect and may have some lubricating and softening actions. It is administered in the form of suppositories, which usually act within 15-30 minutes. It is a useful treatment for babies and young children. Glycerol is inactive by mouth, as it is readily absorbed and extensively metabolised in the liver.