Laxatives

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. Often, taking powerful laxatives can cause diarrhea, accompanied by massive flatulence attacks, due to the overworking of the bowels. Some people who experienced these side effects produced more than three times the average daily amount of flatulence in just one bowel release.

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.

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.

Tobacco smoking has a laxative effect.

Bulk-producing agents

 * Site of Action: Small and large intestine
 * Onset of Action: 12 - 72 hours
 * Examples: psyllium husk (Metamucil), methylcellulose (Citrucel), polycarbophil, dietary fiber, apples

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. 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
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.
 * Site of Action: Small and large intestine
 * Onset of Action: 12 - 72 hours
 * Examples: docusate (Colace, Diocto)

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.

Saline

 * Site of Action: Small and large intestine
 * Onset of Action: 0.5 - 6 hours
 * Examples: Dibasic sodium phosphate, magnesium citrate, magnesium hydroxide (Milk of magnesia), magnesium sulfate, monobasic sodium phosphate, sodium biphosphate, Epsom salt

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.

Sulfate salts are considered the most potent.

Hyperosmotic agents

 * Site of Action: Colon
 * Onset of Action: 0.5 - 3 hours
 * Examples: Glycerin suppositories, Sorbitol, Lactulose, and Polyethylene glycol (PEG).

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.

Solutions of polyethylene glycol and electrolytes (sodium chloride, sodium bicarbonate, potassium chloride, and sometimes sodium sulfate) are used for whole bowel irrigation, a process designed to prepare the bowel for surgery or colonoscopy and to treat certain types of poisoning. Brand names for these solutions include GoLytely, GlycoLax, CoLyte, NuLytely, and others.

Effectiveness
For adults, a randomized controlled trial found greater improvement from 2 sachets (26 grams) of PEG [MiraLax or GlycoLax] versus or 2 sachets (20 grams) of lactulose. 17 grams/day of PEG has been effective and safe in a randomized controlled trial for six months. Another randomized controlled trial found no difference between sorbitol and lactulose.

For children, PEG was found to be more effective than lactulose.

Stimulant / Irritant

 * Site of Action: Colon
 * Examples:

These stimulate peristaltic action and can be dangerous under certain circumstances. Long term use can lead to 'cathartic colon'. 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.

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

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