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A laxative is a preparation used for encouraging defecation, or the expulsion of feces. Laxatives are most often taken to treat constipation. Certain stimulant, lubricant, and saline laxatives are used to evacuate the colon for rectal and bowel examinations. They are sometimes supplemented by enemas.

Laxatives are often abused by bulimics or anorexics (nervosa). Laxative abuse is potentially serious since it can lead to intestinal paralysis, Irritable Bowel Syndrome (IBS), pancreatitis, and other problems.

There are several types of laxatives, listed below. Some laxatives combine more than one type of active ingredient to produce a combination of the effects mentioned. Laxatives may be oral or in suppository form.

While there is no sex difference in constipation from a medical point of view, advertisers tend to promote some brands as being "More for a lady" than others.

Bulk-producing agents

  • Site of Action: Small and large intestine
  • Onset of Action: 12 - 72 hours

Also known as bulk-forming or bulking agents, these include dietary fiber. Bulk-producing agents cause the stool to be bulkier and to retain more water, as well as forming an emollient gel, making it easier for peristaltic action to move it along. Examples: psyllium husk (Metamucil), methylcellulose (Citrucel), polycarbophil, apples. They should be taken with plenty of water. Bulk-producing agents have the gentlest of effects among laxatives and can be taken just for maintaining regular bowel movements.

Stool softeners / Surfactants

  • Site of Action: Small and large intestine
  • Onset of Action: 12 - 72 hours

These cause water and fats to penetrate the stool, making it easier to move along. Many of these quickly produce a tolerance effect and so become ineffective with prolonged use. Their strength is between that of the bulk producers and the stimulants, and they can be used for patients with occasional constipation or those with anorectal conditions for whom passage of a firm stool is painful. Stool softeners include docusate (Colace, Diocto).

Lubricants / Emollient

  • Site of Action: Colon
  • Onset of Action: 6 - 8 hours

These simply make the stool slippery, so that it slides through the intestine more easily. An example is mineral oil, which also retards colonic absorption of water, softening the stool. Mineral oil may decrease the absorption of fat-soluble vitamins (A, D, E and K).

Hydrating agents (osmotics)

These cause the intestines to concentrate more water within, softening the stool. There are two principal types, saline and hyperosmotic. Examples: Milk of Magnesia, Epsom salt.


  • Site of Action: Small and large intestine
  • Onset of Action: 0.5 - 3 hours

Saline laxatives attract and retain water in the intestinal lumen, increasing intraluminal pressure and thus softening the stool. They will also cause the release of cholecystokinin, which stimulates the digestion of fat and protein. Saline laxatives may alter a patient's fluid and electrolyte balance. Examples: Dibasic sodium phosphate, magnesium citrate, magnesium hydroxide (Milk of magnesia), magnesium sulfate, monobasic sodium phosphate, sodium biphosphate. Sulfate salts are considered the most potent.

Hyperosmotic agents

  • Site of Action: Colon
  • Onset of Action: 0.5 - 3 hours

Hyperosmotic laxatives include Glycerin suppositories and Lactulose. Lactulose works by the osmotic effect, which retains water in the colon, lowering the pH and increasing colonic peristalsis. Lactulose is also indicated in Portal-systemic encephalopathy. Glycerin suppositories work mostly by hyperosmotic action, but also the sodium stearate in the preparation causes local irritation to the colon.

Stimulant / Irritant

  • Site of Action: Colon

These stimulate peristaltic action, and can be dangerous under certain circumstances. Stimulant laxatives act on the intestinal mucosa or nerve plexus, they also alter water and electrolyte secretion. They are the most severe among laxatives and should be used only in extreme conditions. Castor oil may be preferred when more complete evacuation is required.

Onset of Action Laxative Name
6 - 8 hours Cascara
Phenolphthalein (phased out because of carcinogenicity concerns)
6 - 10 hours Bisacodyl tablets (Dulcolax)
Senna (Ex-lax)
Aloe Vera
2 - 6 hours Castor oil
15 min - 1 hour Bisacodyl suppository

Castor oil

  • Site of Action: Small intestine
  • Onset of Action:

Castor oil acts directly on intestinal mucosa or nerve plexus and alters water and electrolyte secretion. It is converted into ricinoleic acid (the active component) in the gut.

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