Laxative

Laxatives are foods, compounds, or drugs taken to induce bowel movements, 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.

The use of laxatives in eating disorders
Laxatives are often abused by people with bulimia nervosa or anorexia nervosa. Laxative abuse is potentially serious since it can lead to intestinal paralysis, Irritable Bowel Syndrome (IBS), pancreatitis, renal failure, and other problems.

Psychological aspects in the addiction to laxatives
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.

Constipation with no known organic cause, i.e. no medical explanation, exhibits gender differences in prevalence, females are more often affected than males. Not surprisingly, some advertisers promote their brands as being more feminine and thereby tailor their message to the market. The way laxatives function in males and females, however, does not exhibit significant differences.

Bulk-producing agents
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.
 * Site of Action: Small and large intestine
 * Onset of Action: 12 - 72 hours

Stool softeners / Surfactants
These cause water & 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).
 * Site of Action: Small and large intestine
 * Onset of Action: 12 - 72 hours

Lubricants / Emollient
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).
 * Site of Action: Colon
 * Onset of Action: 6 - 8 hours

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.

Saline
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.
 * Site of Action: Small and large intestine
 * Onset of Action: 0.5 - 6 hours

Hyperosmotic agents
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. According to trials, Polyethylene glycol was found to be more effective than lactulose. 
 * Site of Action: Colon
 * Onset of Action: 0.5 - 3 hours

Stimulant / Irritant
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.
 * Site of Action: Colon

Examples:

Castor oil
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.
 * Site of Action: Small intestine

Other
Tegaserod is a motility stimulant that works through activation of 5-HT4 receptors of the enteric nervous system in the gastrointestinal tract.