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Osmotic laxative

Polyethylene glycol

Polyethylene glycol (PEG) is an osmotic laxative that draws water into the bowel lumen, softening stool and promoting evacuation. It is the first-line agent for constipation management and bowel preparation before colonoscopy. PEG is non-absorbed, acts locally in the colon, and has minimal systemic effects.

Practice questionsHigh-yield facts
Drug classOsmotic laxative (inert polymer)
MechanismRetains water in bowel lumen via osmotic effect
Main usesConstipation, bowel prep for colonoscopy
Key adverse effectAbdominal bloating, cramping, mild nausea
4 min readFor medical studentsUpdated 2026-10-15

Mechanism of action

Polyethylene glycol is a non-absorbed, inert polymer that functions as an osmotic laxative. It dissolves in water and draws fluid into the intestinal lumen via osmotic pressure, increasing stool bulk and water content. This softens stool consistency and stimulates peristalsis, promoting bowel evacuation within 24–72 hours.

PEG has negligible systemic absorption. It does not interact with gut mucosa receptors, enzymes, or transporters. Its action is purely physical and osmotic, making it one of the safest laxatives available.

High-yield for exams
  • PEG is an osmotic laxative; it draws water into the bowel and is NOT systemically absorbed.
  • First-line agent for chronic constipation and the gold standard for colonoscopy bowel prep.
  • Onset of action varies: high-dose bowel prep works within 1–3 hours; daily doses for constipation take 24–72 hours.
  • Minimal drug interactions; safe in renal impairment, pregnancy, and elderly patients.
  • Common side effects: bloating, cramping, nausea — generally mild and self-limited.
  • Contraindicated in bowel obstruction, perforation, and toxic megacolon.
  • Superior tolerability compared to lactulose (less gas, fewer cramps).
  • Dosing follows product label; no standard fixed dose across all indications.

Indications

  • Chronic idiopathic constipation — first-line agent for adults and children >6 months
  • Bowel preparation — standard agent before colonoscopy, sigmoidoscopy, or barium studies
  • Opioid-induced constipation — effective adjunct or alternative to stool softeners
  • Fecal impaction — used to soften impacted stool prior to manual disimpaction
  • Post-surgical constipation — safe option after abdominal surgery

PEG is well-tolerated in elderly patients and those with renal or hepatic impairment due to minimal absorption.

Pharmacokinetics

Absorption: Negligible. PEG is not absorbed across the intestinal mucosa.

Distribution: N/A — remains in the gastrointestinal tract.

Metabolism: No metabolism occurs.

Elimination: Excreted unchanged in feces within 24–72 hours. Small intestinal transit time is typically 1–2 hours; colonic transit varies.

Onset: Varies by dose and indication. High-dose regimens for bowel preparation take effect within 1–3 hours. Standard daily doses for chronic constipation typically take 24–72 hours to produce a bowel movement and achieve full therapeutic effect.

Duration: Effects persist while PEG remains in the bowel; typically resolved within 72 hours of last dose.

PEG is available in powder form mixed with water (often as 3350, 4000, or higher molecular weight formulations). Dosing follows the product label and varies by indication and patient age.

Adverse effects

Common (>10%):

  • Abdominal bloating and distension
  • Mild cramping or discomfort
  • Nausea
  • Flatulence

Less common (1–10%):

  • Vomiting
  • Mild headache
  • Increased appetite after bowel evacuation

Rare (<1%):

  • Allergic reaction (very rare)
  • Dehydration if fluid intake is inadequate
  • Electrolyte abnormalities (only if large volumes used repeatedly without adequate hydration)

Gastrointestinal perforation has been reported in patients with severe colonic disease (toxic megacolon, inflammatory bowel disease exacerbation) but is extremely rare with standard use.

Contraindications and warnings

Contraindications:

  • Gastrointestinal obstruction or ileus
  • Perforation or suspected perforation of the bowel
  • Toxic megacolon (absolute contraindication)
  • Severe dehydration
  • Gastric retention

Cautions and warnings:

  • Inflammatory bowel disease: Use with caution during acute exacerbations; risk of perforation is theoretical but real. Consult gastroenterology.
  • Renal impairment: No dose adjustment needed; PEG is not absorbed. However, ensure adequate fluid intake.
  • Pregnancy: Generally considered safe (Category B); no systematic toxicity data but minimal absorption reduces risk.
  • Breastfeeding: Safe; PEG is not absorbed and does not enter milk.
  • Dehydration: Ensure adequate oral fluid intake, especially in elderly or those on diuretics.
  • Electrolyte abnormalities: Monitor in patients receiving repeated or prolonged use, particularly if comorbid renal disease exists.

Drug interactions

PEG has minimal drug interactions due to negligible systemic absorption.

Notable considerations:

  • Oral medications: PEG may reduce absorption of concurrently ingested oral medications by increasing stool transit time and dilution. Separate administration by 2 hours when possible.
  • Diuretics and ACE inhibitors: No direct interaction, but ensure adequate fluid intake to avoid dehydration.
  • NSAIDs: No direct interaction; however, PEG-induced fluid shifts may theoretically affect renal perfusion in susceptible patients. Monitor renal function in elderly patients or those with CKD.
  • Antibiotics: No clinically significant interaction; separate dosing by 2 hours if constipation is a concern.

PEG does not inhibit or induce cytochrome P450 enzymes and does not bind significantly to other medications.

Clinical pearls

  • First-line for constipation: PEG is preferred over lactulose for most patients due to better tolerability and fewer side effects (less bloating and cramping than lactulose).
  • Bowel prep standard: PEG is the gold standard for colonoscopy preparation; higher molecular weight formulations (PEG 3350, 4000) are preferred for efficacy and safety.
  • Safe in renal disease: Unlike some osmotic laxatives, PEG does not accumulate or cause hypermagnesemia; ideal for CKD patients.
  • Pediatric use: Approved and safe in children >6 months; dosing is lower and adjusted by age/weight per product labeling.
  • Opioid-induced constipation: More effective than stool softeners alone; often combined with stimulant laxatives for severe cases.
  • Predictable onset: High-dose regimens work within 1–3 hours for bowel prep; standard daily use takes 24–72 hours, making timing appropriate to clinical context.
  • No systemic side effects: Absence of absorption means no risk of systemic toxicity, electrolyte derangement with short-term use, or drug accumulation.
  • Patient education: Mixing with flavored beverages improves palatability and adherence; adequate hydration is essential during use.
Mnemonic

PEG = Pulls water, Evacs bowel, Generally safe

Practice questions

1. A 67-year-old man with type 2 diabetes and CKD stage 3b (eGFR 35 mL/min) presents with constipation for 3 weeks. He is on lisinopril, metformin, and acetaminophen. Which laxative is safest for this patient and why?

Answer: A. PEG is ideal for renal disease because it is not absorbed and does not accumulate systemically. Magnesium hydroxide (option 2) is contraindicated in CKD due to risk of hypermagnesemia from absorption. Lactulose (option 1) can accumulate and cause electrolyte issues. Bisacodyl and docusate are less effective for constipation. The key testable fact is that PEG's non-absorbable nature makes it uniquely safe in renal impairment.

2. A 52-year-old woman is scheduled for colonoscopy tomorrow. Her gastroenterologist prescribes polyethylene glycol 3350 for bowel prep. She reports taking metoprolol and omeprazole daily. What is the most important patient education point regarding drug interactions?

Answer: B. PEG has minimal systemic drug interactions due to non-absorption, but it may reduce absorption of concurrently ingested oral medications by increasing transit time. Separating administration by 2 hours is a practical strategy. Options 0, 2, and 4 are false; PEG does not interact with metoprolol or omeprazole. Option 3 is incorrect because omeprazole does not increase PEG side effects. This tests understanding of PEG's local, non-absorbed mechanism.

3. A 78-year-old man with a history of ulcerative colitis in remission presents with constipation. He has not had a flare in 2 years. His gastroenterologist prescribes polyethylene glycol. Which statement correctly reflects the safety profile in this scenario?

Answer: B. PEG is not absolutely contraindicated in IBD remission (option 0 is too broad), but caution is warranted because perforation risk exists during active disease exacerbation. In remission, PEG can be used if monitored. Options 2, 3, and 4 are clinically incorrect. This tests nuanced understanding of contraindications and the distinction between absolute and relative cautions. The key is that IBD in remission is a relative, not absolute, contraindication.

Frequently asked questions

How does polyethylene glycol work?

PEG is an osmotic laxative that dissolves in water and draws fluid into the intestinal lumen. This increases stool bulk and water content, softening stool and stimulating bowel evacuation. High-dose regimens work within 1–3 hours; standard daily use takes 24–72 hours. PEG is not absorbed systemically and acts only locally in the colon.

What are the main side effects of polyethylene glycol?

Common side effects include abdominal bloating, cramping, nausea, and flatulence. These are usually mild and self-limited. Severe side effects are rare; perforation has been reported only in patients with severe colonic disease or obstruction, which are contraindications to use.

Is polyethylene glycol safe in pregnancy?

Yes, PEG is generally considered safe in pregnancy (Category B). It is not absorbed systemically and does not enter fetal circulation. It is also safe during breastfeeding. Always consult obstetrics for individualized guidance in high-risk pregnancies.

Can polyethylene glycol be used in patients with kidney disease?

Yes, PEG is safe in renal impairment because it is not absorbed and does not accumulate. No dose adjustment is needed. However, ensure adequate oral hydration, as PEG draws fluid into the bowel and may contribute to dehydration if intake is inadequate.

Polyethylene glycol versus lactulose — what is the difference?

Both are osmotic laxatives, but PEG is preferred for most patients. PEG has faster onset (1–3 hours for high-dose prep vs. 6–12 hours), fewer side effects (less gas and cramping), and works better for bowel prep. Lactulose is fermented by colonic bacteria, causing bloating; PEG is inert and non-fermentable.

Conditions it is used for

Chronic idiopathic constipationOpioid-induced constipationFecal impactionPost-surgical constipationIrritable bowel syndrome with constipationBowel preparation for colonoscopyBowel preparation for sigmoidoscopyDrug-induced constipation
LactuloseBisacodylDocusateMagnesium citrateMagnesium hydroxideSennaLinaclotideLubiprostone

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: A06AD15

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.