Polyethylene Glycol

Polyethylene glycol (macrogol) is a long-chain, inert, non-absorbable osmotic laxative. It holds water in the gut lumen, softening stool and increasing stool volume, without fermentation or gas. It is first-line for chronic constipation, faecal impaction and bowel preparation.

Classification. Osmotic laxative.

Mechanism. Molecules of polyethylene glycol (usually molecular weight 3350 or 4000) bind water by hydrogen bonds, so the water stays in the faecal mass. Preparations containing electrolytes limit net sodium and potassium shifts across the colon.

Pharmacodynamics. Effect begins in 24 to 48 hours. Stool consistency is more predictable than with lactulose and there is less bloating.

Pharmacokinetics. Less than 0.3% is absorbed. It is excreted unchanged in faeces, and the absorbed fraction in urine.

Adult oral doses unless stated.

Chronic constipation.

• Polyethylene glycol 3350 without electrolytes: 17 g once daily dissolved in 125 to 250 mL of water; adjust to one soft stool daily.

• Compound preparations with electrolytes (13.8 g sachet): 1 to 3 sachets daily in divided doses, each dissolved in 125 mL water; maintenance 1 to 2 sachets daily.

• Review need after 2 weeks.

Faecal impaction.

• Compound preparation with electrolytes: 8 sachets (13.8 g each) dissolved in 1 litre of water and drunk within 6 hours, for up to 3 days.

• Not for patients with cardiovascular or kidney impairment; use in hospital.

• Follow with maintenance and stimulant laxative if needed.

Bowel preparation before colonoscopy.

• High-volume solution (polyethylene glycol with electrolytes) 4 litres. Split dose: 2 litres in the evening before and 2 litres on the morning of the procedure, drinking about 250 mL every 10 to 15 minutes.

• Finish at least 2 hours before the procedure.

• A low-volume 2 litre preparation with an adjunct is an alternative.

Children (functional constipation).

• Maintenance: 0.4 to 0.8 g/kg once daily, not exceeding the adult dose.

• Disimpaction: 1 to 1.5 g/kg once daily for 3 days (maximum 100 g daily) under supervision.

• Age under 1 year: specialist use.

Kidney and liver adjustment. Polyethylene glycol alone needs no adjustment. In kidney impairment or heart failure avoid large-volume electrolyte preparations.

Contraindications. Intestinal obstruction, perforation or ileus; toxic colitis and megacolon; hypersensitivity. Caution in patients with impaired swallowing, reduced consciousness or risk of aspiration.

Common adverse effects. Bloating, abdominal cramp, nausea, flatulence and diarrhoea.

Serious adverse effects.

• Dehydration, hyponatraemia and hypokalaemia with large-volume bowel preparation, particularly in older adults, kidney impairment and heart failure.

• Aspiration of the solution in patients with swallowing problems.

• Rare anaphylaxis and angioedema.

• Mallory-Weiss tear from forceful vomiting.

Major interactions.

• Oral medicines taken within 1 hour of bowel preparation may be flushed through unabsorbed; separate by at least 1 hour.

• Thickening agents based on xanthan gum lose effect when mixed with macrogol; do not combine in dysphagia.

• Diuretics and NSAIDs: increase the risk of electrolyte disturbance and kidney injury with bowel preparation.

• Drugs causing hyponatraemia (SSRIs, carbamazepine, desmopressin): additive risk.

Special groups.

• Pregnancy: not absorbed; compatible.

• Breastfeeding: compatible.

• Children: first-line osmotic laxative; weight-based dosing.

• Older adults: monitor fluids and electrolytes with bowel preparation; consider admission for frail patients.

• Kidney impairment: avoid large-volume preparations with electrolytes except under supervision.

• Liver impairment: no adjustment; prevent dehydration.

• Regional: widely available in urban centres; may be costly in rural areas.

Formulations. Powder sachets (13.8 g or 17 g); powder for large-volume bowel preparation; oral solution.

Administration. Dissolve each dose fully in the specified volume of water, juice or squash. Drink it all. Drink additional clear fluids. Sachets must be taken in divided doses and not all together unless in the impaction regimen.

Baseline. Hydration, kidney function and electrolytes in older adults or heart and kidney disease before bowel preparation.

Monitoring.

• Stool frequency and consistency.

• Hydration, sodium and potassium after bowel preparation in at-risk patients.

• Abdominal distension and vomiting during impaction treatment.

Therapeutic drug monitoring. Not applicable.

Stopping. Reduce the dose if stool becomes watery. Maintenance should continue for at least 2 months after normal bowel habit returns in children, and be reduced gradually.

Referral and escalation. Abdominal examination and imaging for suspected obstruction. Colorectal referral for red flags: weight loss, rectal bleeding, iron deficiency anaemia or change in bowel habit over age 50.