Type of laxative		Mechanism of action	Onset of action	Potential adverse effects
Bulk laxative		Increases fecal bulk as well as the fluid retained in the bowel lumen	12 to 24 hours or more	Increased gas; bloating; bowel obstruction if strictures present; choking if powder forms are not taken with enough liquid
	Psyllium seed
	Bran
	Calcium polycarbophil
Emollients and stool softeners		Lubricates and softens fecal mass	24 to 48 hours	Minor effects such as bitter taste and nausea
	Dioctyl sodium
	Calcium sulfosuccinate (docusate sodium)
Stimulants and irritants		Alters intestinal mucosal permeability Stimulates muscle activity and fluid secretions	10 minutes (sodium bicarbonate plus potassium bitartrate suppository [Ceo-two]) to 12 hours	Dermatitis; electrolyte imbalance; melanosis coli
	Phenolphthalein*
	Bisacodyl
	Senna
	Cascara
	Sodium bicarbonate plus potassium bitartrate
Osmotic laxative		Salts lead to retained fluid in the bowel lumen, with a net increase of fluid secretions in the small intestines	2 to 48 hours	Electrolyte imbalance; excessive gas; hypermagnesemia, hypocalcemia and hyperphosphatemia in patients with renal failure; dehydration
	Ricinoleic acid
	Lactulose
	Magnesium salts
	Sodium salts
	Sorbitol
Enema		Causes reflex evacuation	Within 30 minutes	Dehydration; hypocalcemia and hyperphosphatemia in patients with chronic renal failure
	Tap water
	Saline
	Sodium phosphate
	Oil
Nonabsorbable solutions		Volume lavage	Within 4 hours	Nausea; abdominal fullness; bloating
	Polyethylene glycol