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)
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
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
Coat the stool with slippery lipids and retard colonic absorption of water so that the stool slides through the colon more easily. Lubricant<unk>  also increase the weight of stool and decrease intestinal transit time.
Site of action: colon
Onset of action: 6–8 hours
Example: mineral oil
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)
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)
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<unk> action by an unknown mechanism.
Site of action: colon
Onset of action: 2–6 hours
Examples: castor oil
Laxatives
Antihistamines
Opioids
Antivirals
Toxic alcohols
Proton-pump inhibitor
Synthetic cannabinoids
Local anesthetics