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Bulk-forming agents

Substances, such as dietary fiber and hydrophilic agents that add bulk and water to more easily through the intestines.

Dietary fiber

Includes insoluble fiber and soluble fiber, such as:

  • Fruits, such as bananas, kiwifruits, prunes, apples (with skin), pears (with skin), and raspberries
  • Vegetables, such as broccoli, string beans, kale, spinach, cooked winter squash, cooked green peas, and baked potatoes (with skin)
  • Whole grains
  • Bran products
  • Nuts
  • Legumes, such as beans, peas, and lentils

Emollient agents (stool softeners)

Anionic surfactants that enable additional water and fats to be incorporated in the stool, making it easier for them to move through the gastrointestinal tract.

  • Site of action: small and large intestines
  • Onset of action: 12–72 hours
  • Examples: docusate (Colace, Diocto), Gibs-Eze

Lubricant agents

Coat the stool with slippery lipids and retard colonic absorption of water so that the stool slides through the colon more easily. Lubricant laxatives also increase the weight of stool and decrease intestinal transit time.

  • Site of action: colon
  • Onset of action: 6–8 hours
  • Example: mineral oil

Hyperosmotic agents

Substances that cause the intestines to hold more water within and create an osmotic effect that stimulates a bowel movement.

  • Site of action: colon
  • Onset of Action: 12–72 hours (oral) 0.25 - 1 hour (rectal)
  • Examples: glycerin suppositories, sorbitol, lactulose, and PEG (Colyte, MiraLax)

Saline laxative agents

Non-absorbable osmotic 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.

Stimulant agents

Act on the intestinal mucosa or nerve plexus, altering water and electrolyte secretion. They also stimulate peristaltic action and can be dangerous under certain circumstances.

  • Site of action: colon
  • Onset of action: 6–10 hours
  • Examples: senna glycoside, bisacodyl


Castor oil is a glyceride that is hydrolyzed by pancreatic lipase to ricinoleic acid, which produces laxative action by an unknown mechanism.

  • Site of action: colon
  • Onset of action: 2–6 hours
  • Examples: castor oil

See Also