Hyperosmotic agents
Hyperosmotic laxatives are substances that cause the intestines to hold more water within and create an osmotic effect that stimulates a bowel movement.Properties

Site of action: colon
Onset of action: 12–72 hours (oral), 0.25–1 hour (rectal)
Examples: glycerin suppositories (Hallens), sorbitol, lactulose, and PEG (Colyte, MiraLax)Lactulose works by the osmotic effect, which retains water in the colon; lowering the pH through bacterial fermentation to lactic, formic and acetic acid; and increasing colonic peristalsis. Lactulose is also indicated in portal-systemic encephalopathy. Glycerin suppositories work mostly by hyperosmotic action, but the sodium stearate in the preparation also 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, Cosmocol, CoLyte, Miralax, Movicol, NuLytely, Suprep, and Fortrans. Solutions of sorbitol (SoftLax) have similar effects. Saline laxative agents
Saline laxatives are non-absorbable osmotically active substances that attract and retain water in the intestinal lumen, increasing intraluminal pressure that mechanically stimulates evacuation of the bowel. Magnesium-containing agents also cause the release of cholecystokinin, which increases intestinal motility and fluid secretion. Saline laxatives may alter a patients fluid and electrolyte balance.
Laxatives, purgatives, or aperients are substances that loosen stools and increase bowel movements. They are used to treat and prevent constipation. Laxatives vary as to how they work and the side effects they may have. Certain stimulant, lubricant and saline laxatives are used to evacuate the colon for rectal and bowel examinations, and may be supplemented by enemas under certain circumstances. Sufficiently high doses of laxatives may cause diarrhea. Some laxatives combine more than one active ingredient. Laxatives may be administered orally or rectally. Types
Bulk-forming agents
Bulk-forming laxatives, also known as roughage, are substances, such as fiber in food and hydrophilic agents in over-the-counter drugs, that add bulk and water to stools so that they can pass more easily through the intestines (lower part of the digestive tract).Properties

Site of action: small and large intestines
Onset of action: 12–72 hours
Examples: dietary fiber, Metamucil, Citrucel, FiberConBulk-forming agents generally have the gentlest of effects among laxatives, making them ideal for long-term maintenance of regular bowel movements. Dietary fiber
Foods that help with laxation include fiber-rich foods.