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Bulk producers

Stool softeners

Osmotic agents

Stimulant laxatives Dietary fibre

Mucilaginous polysaccharides

Methylcellulose

Synthetic surface active agents, liquid paraffin

Sodium, potassium and magnesium salts

Sodium picosulphate, glycerine Bran, wholemeal bread, Fybogel (ispaghula), Normacol (sterculia)

Agarol, Dioctyl, Petrolager, Milpar

Magnesium sulphate, milk of magnesia, lactulose

Senna, Senokot, bisacodyl, Dulcolax, co-danthrusate, Picolax, glycerol

Bulking agents

Bulking agents are usually the first line of laxative treatment and work by retaining water and promoting microbial growth in the colon, increasing faecal mass and stimulating peristalsis (Peate 2003). Ispaghula husk (Isogel, Regulan) and sterculia (Normacol) both trap water in the intestine by the formation of a highly viscous gel which softens faeces, increases weight and reduces transit time (Butler 1998). These agents need plenty of fluid in order to work (2–3 litres per day), so their appropriateness should be questioned for those with advanced cancer or for the older patient (Hinrichs and Huseboe 2001, Maestri-Banks 1998, Perdue 2005). They also take a few days to exert their effect (Maestri-Banks 1998) and so are not suitable to relieve acute constipation. Also, they are contraindicated in some patients, including those who have bowel obstruction, faecal impaction, acute abdominal pain and reduced muscle tone, or those who have had recent bowel surgery. Increasing the bulk may worsen impaction, lead to increased colonic faecal loading or even intestinal obstruction (Norton 1996a), and in some cases may increase the risk of faecal incontinence. Other potentially harmful effects include malabsorption of minerals, calcium, iron and fat-soluble vitamins, and reduced bio-availability of some drugs (Taylor 1997). Another problem initially is that bulk laxatives tend to distend the abdomen, often making the individual feel full and uncomfortable. Sometimes this leads to temporary anorexia (Taylor 1997).

Stool softeners

Stool-softening preparations, such as docusate sodium and glycerol (glycerine) suppositories, act by lowering the surface tension of faeces which allows water to penetrate and soften the stool (Hinrichs and Huseboe 2001, Peate 2003). They may also have a weak stimulatory effect (Barrett 1992), but drugs of this type are often given in combination with a chemical stimulant (Shepherd 2000). Softening agents usually take 1–3 days to work (Day 2001).

Liquid paraffin acts as a lubricant as well as a stool softener by coating the faeces and allowing easier passage. However, its use should be avoided as there are a number of problems associated with this preparation. It interferes with the absorption of fat-soluble vitamins and can also cause skin irritation and changes to the bowel mucosa while accidental inhalation of droplets of liquid paraffin may result in lipoid pneumonia (BNF 2011, Maestri-Banks 1998, Peate 2003).

Osmotic agents

These can be divided into two subgroups: lactulose and magnesium preparations.

Lactulose is a synthetic disaccharide which exerts an osmotic effect in the small bowel. Distension of the small bowel induces propulsion which in turn reduces transit time. Colonic bacteria metabolize lactulose into a short-chain organic salt which is then absorbed; therefore the osmotic effect does not continue throughout the colon (Barrett 1992). This process of metabolism also produces gas which in turn stimulates colonic movements and increases bacterial growth. This results in increased stool weight and thus colonic transit time is shortened (Spiller 1994). However, bowel action may still take up to 3 days to occur following administration (Shepherd 2000) and flatulence, cramps and abdominal discomfort are associated with high dosages.

Magnesium preparations also exert an osmotic effect on the gut and additionally they stimulate the release of cholecystokinin. This encourages intestinal motility and fluid secretion (Nathan 1996). They have a rapid effect, working within 2–6 hours,