Osmotic Laxatives

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These are but little absorbed and increase the bulk and reduce viscosity of intestinal contents to promote a fluid stool.

Some inorganic salts retain water in the intestinal lumen or, if given as hypertonic solution, withdraw it from the body. When constipation is mild, magnesium hydroxide will suffice but magnesium sulphate (Epsom4 salts) is used when a more powerful effect is needed. Both magnesium salts act in 2-4 h. The small amount of magnesium absorbed when the sulphate is frequently used can be enough to cause magnesium poisoning in patients with renal impairment, the central nervous effects of which somewhat resemble those of uraemia. Magnesium sulphate 50% (hypertonic) is available as a single dose retention enema to reduce cerebrospinal fluid pressure in neurosurgery.

Lactulose is a synthetic disaccharide. Taken orally, it is unaffected by small intestinal disaccharidases, is not absorbed and thus acts as an osmotic laxative. Tolerance may develop. Lactulose is also used in

3 Named after Sterculinus, a god of ancient Rome, who presided over manuring of agricultural land.

4 Epsom, a town near London, known for its now defunct mineral spring water, and for horse racing.

the treatment of hepatic encephalopathy (see Chapter 33).

Osmotic laxatives are frequently used to clear the colon for diagnostic procedures or surgery. Enemas containing phosphate or citrate effectively evacuate the distal colon and can be useful for treating obstinate constipation in elderly or debilitated patients. Oral preparations containing magnesium sulphate and citric acid (Citramag) or polyethylene glycol (Klean Prep) are used in preparation for colonoscopy; they are made up with water to create an isotonic solution and some patients find the large volumes difficult to tolerate. Isotonic mannitol was used for the same purpose in the early days of colonoscopy, but has since been abandoned; hydrogen liberated by the action of colonic bacteria was the cause of several intestinal explosions triggered by the use of diathermy. The stimulant laxative sodium picosulphate (Picolax) is a frequently used alternative to the osmotic preparations. Care should be used with all these preparations in the elderly; they can induce dehydration, hypovolemia and electrolyte disturbances.


The softening properties of these agents are useful in the management of anal fissure (see below) and haemorrhoids.

Docusate sodium (dioctyl sodium sulphosuccinate) softens faeces by lowering the surface tension of fluids in the bowel. This allows more water to remain in the faeces. It appears also to have bowel stimulant properties but these are relatively weak. Docusate sodium acts in 1-2 days. Poloxamers, e.g. poloxalkol (poloxamer 188), act similarly and are used in combination with other agents.

Liquid paraffin is a chemically inert mineral oil and is not digested. It promotes the passage of softer faeces. It is often presented in emulsions with magnesium hydroxide. Large doses may leak out of the anus causing both physical and social discomfort. Paraffin taken orally over long periods, especially at night, may be aspirated and cause chronic lipoid pneumonia. An unusual case resulted from attempts by a patient, an amateur singer, to lubricate his larynx with liquid paraffin. Because of

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Constipation Prescription

Constipation Prescription

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