Laxatives

Types

Bulk-forming agents

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

• Site of action: Small and large intestines
• Onset of action: 12–72 hours
• Examples: dietary fiber, psyllium (Metamucil), methylcellulose (Citrucel), polycarbophil (FiberCon)

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.

• Site of action: small and large intestines
• Onset of action: 0.5–3 hours (oral), 2–15 minutes (rectal)
• Examples: sodium phosphate (and variants), magnesium citrate, magnesium hydroxide (milk of magnesia), and magnesium sulfate (Epsom salt)

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, bisacodyl

They are the most powerful among laxatives and should be used with care. Prolonged use of stimulant laxatives can create drug dependence by damaging the colon's haustral folds, making a user less able to move feces through the colon on their own. A study of patients with chronic constipation found that 28% of chronic stimulant laxative users lost haustral folds over the course of one year, while none of the control group did.

Miscellaneous

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

Properties

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

Long-term use of castor oil may result in loss of fluid, electrolytes, and nutrients.

Serotonin agonist

These are motility stimulants that work through activation of 5-HT₄ receptors of the enteric nervous system in the gastrointestinal tract. However, some have been discontinued or restricted due to potentially harmful cardiovascular side-effects.

Tegaserod (brand name Zelnorm) was removed from the general U.S. and Canadian markets in 2007, due to reports of increased risks of heart attack or stroke. It is still available to physicians for patients in emergency situations that are life-threatening or require hospitalization.

Prucalopride (brand name Resolor) is a current drug approved for use in the EU October 15, 2009 and in Canada (brand name Resotran) on December 7, 2011. It has not been approved by the Food and Drug Administration for use in the United States, but it is in development by Shire PLC.

Chloride channel activators

Lubiprostone is used in the management of chronic idiopathic constipation and irritable bowel syndrome. It causes the intestines to produce a chloride-rich fluid secretion that softens the stool, increases motility, and promotes spontaneous bowel movements (SBM).