Pharmacology · Class
Laxatives !
Medicines that soften stool or speed up the bowel. They work by drawing in water, coating stool, stimulating the gut wall or blocking opioid effects.
Updated 2026-11-01
What they are
A broad group for constipation and bowel preparation: osmotic agents (lactulose, macrogol), stimulants (bisacodyl, senna), stool softeners (docusate), chloride-secretion activators (lubiprostone, linaclotide) and peripheral μ-opioid antagonists (methylnaltrexone).
Mechanism of action
Osmotic laxatives are poorly absorbed molecules that hold water in the lumen. Stimulants act on the enteric nerves and the colonic lining to increase motility and fluid secretion. Softeners let water and fat into the stool. Secretagogues open chloride channels in the gut lining so water follows. Peripheral opioid antagonists block μ receptors in the gut wall without reversing pain relief in the brain.
Target: Several: colonic contents, enteric nervous system, chloride channels (ClC-2, CFTR via guanylate cyclase C) and gut μ-opioid receptors
Drugs in this class
- LactuloseA sugar the gut cannot absorb: it softens stool and, in liver disease, traps ammonia in the colon to prevent and treat…Colonic contents (osmotic effect and ammonia trapping by colonic bacteria)
- Macrogol (polyethylene glycol)An inert, non-absorbed polymer that holds water in the bowel and softens stool. Also used, with salts, to clean the…Colonic lumen (osmotic water retention)
- BisacodylA stimulant laxative that makes the bowel contract. Tablets act overnight, suppositories much faster; also used to…Enteric nervous system and colonic mucosa
- SennaA plant-based stimulant laxative: its sennosides are activated in the colon and make it contract, usually overnight.Colonic mucosa and enteric nervous system
- Docusate sodiumA stool softener: a detergent-like molecule that lets water and fat mix into hard stool so it passes with less…Stool surface (surfactant action) and intestinal mucosa
- LinaclotideA small peptide that acts on the gut lining to release chloride and water into the bowel. Used for…Guanylate cyclase C (GC-C) on the luminal surface of intestinal cells
- LubiprostoneA prostaglandin-derived drug that opens chloride channels in the gut lining, so fluid enters the bowel and stool…ClC-2 chloride channel on the apical membrane of intestinal cells
- MethylnaltrexoneAn opioid blocker that stays outside the brain: it reverses opioid constipation in the gut without taking away pain…μ-opioid receptors of the gut (peripheral)
Others in the class
- Psyllium
- Lactitol
- Sorbitol
- Magnesium hydroxide
- Sodium picosulfate
- Prucalopride
- Plecanatide
- Naloxegol
- Naldemedine
Test yourself
Think of the answer, then tap the question to see it.
Which laxative is also used to lower blood ammonia in hepatic encephalopathy?
Lactulose: colonic bacteria turn it into acids that trap ammonia as ammonium, which then leaves in the stool.
How do lubiprostone and linaclotide differ in mechanism?
Lubiprostone activates the ClC-2 chloride channel directly; linaclotide activates guanylate cyclase C, and the rise in cGMP opens CFTR. Both increase chloride-rich fluid secretion.
What brown discoloration of the colon is linked to chronic senna use?
Melanosis coli, a pigment in the colonic lining after long-term anthraquinone laxatives.
Why can methylnaltrexone relieve opioid constipation without causing pain to return?
It is a quaternary amine that barely crosses the blood-brain barrier, so it blocks μ receptors in the gut but not in the brain.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 62 (McGraw Hill, 2025); Polyethylene Glycol 3350: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2016); Bisacodyl: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2018).
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Representative model, for educational purposes only.