• Bulk-forming laxatives (ispaghula husk, methylcellulose, and sterculia) act by retaining fluid within the stool and increasing faecal mass, leading to stimulation of peristalsis. They also have stool-softening properties.
• Osmotic laxatives (e.g. lactulose, macrogols, phosphate enemas, and sodium citrate enemas) act by increasing the amount of fluid in the large bowel, by retaining fluid in the bowel, and by drawing fluid from the body into the bowel. Fluid accumulation in the lower bowel produces distension, leading to stimulation of peristalsis. Lactulose and macrogols also have stool-softening properties.
• Stimulant laxatives cause peristalsis by stimulating colonic nerves (senna) or colonic and rectal nerves (bisacodyl, sodium picosulfate).
• Senna is hydrolyzed to the active metabolite by bacterial enzymes in the large bowel.
• Bisacodyl and sodium picosulfate are hydrolyzed to the same active metabolite. However, bisacodyl is hydrolyzed by intestinal enzymes, whilst sodium picosulfate relies on colonic bacteria.
• Surface-wetting agents (docusate and poloxamer [an active ingredient of co-danthramer]) reduce the surface tension of the stool, allowing water to penetrate and soften it. Docusate also has a relatively weak stimulant effect.
• Glycerol suppositories act as a lubricant, and have a weak stimulant action (probably due to an irritant effect).
• Arachis oil enemas lubricate and soften faeces, thereby promoting a bowel movement. Bulk laxatives, osmotic laxatives, and docusate also have softening properties.

[Micromedex, 2007; Twycross and Wilcock, 2007]