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 eating disorders. Laxative abuse is potentially serious since it can lead to intestinal paralysis,

Bulk-producing agents
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) Stool softeners / Surfactants
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 Lubricants / Emollient
These cause the intestines to concentrate more water within, softening the stool. There are two principal types, saline and hyperosmotic.

Hydrating agents (osmotics)
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.

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
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.

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

Site of Action: Colon
Examples: Stimulant / Irritant
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