Class Explorer · Gastrointestinal
Laxatives
Soften stool, draw water in, or stimulate propulsion. The mechanism decides the pairing: a stimulant for opioid constipation, an osmotic for hard stool.
Members
Polyethylene glycol 3350An inert molecule that holds water and does nothing else.Senna (sennosides)The stimulant that opioid constipation actually needs.LactuloseAn osmotic laxative that doubles as an ammonia trap.
Also in this class: Docusate · Bisacodyl · Magnesium hydroxide. Full pages arrive as the library grows.
Compare the members
Polyethylene glycol 3350
- Mechanism
- Osmotic, inert
- Best for
- First-line chronic constipation
Senna (sennosides)
- Mechanism
- Stimulant
- Best for
- Opioid-induced constipation
Lactulose
- Mechanism
- Osmotic, fermented
- Best for
- Hepatic encephalopathy
Shared across the class
- Indications
- Constipation · Hepatic encephalopathy (lactulose) · Bowel preparation
- Adverse effects
- Cramping · Diarrhoea · Electrolyte disturbance with overuse
- Contraindications
- Bowel obstruction
- Monitoring
- Stool frequency and form · Electrolytes with high-volume or prolonged use · In encephalopathy: aim for two to three soft stools daily, titrated to mental state
What makes each agent different
- Polyethylene glycol is an inert osmotic agent, not absorbed and not fermented: the best-evidenced first-line laxative and the gentlest.
- Senna is a stimulant acting on the myenteric plexus, which is exactly what opioid-induced constipation needs, because the problem there is stopped propulsion rather than hard stool.
- Lactulose is an osmotic that is also fermented, which produces the gas and bloating patients complain about, and is the same fermentation that acidifies the colon and traps ammonia in encephalopathy.
- Docusate has repeatedly failed to beat placebo and mostly survives out of habit.
Opioid constipation needs a stimulant, started with the opioid rather than after the problem appears.
Lactulose in encephalopathy is dosed against the number of stools, not against a milligram target, and the family should know that.
Last reviewed 2026-08-16 · Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed.