Laxatives

Laxatives are drugs that soften stool, increase its water content or stimulate colonic motility to relieve constipation, empty the bowel before procedures and lower ammonia absorption in hepatic encephalopathy. Chronic constipation means fewer than three complete bowel movements a week, straining, hard stool or incomplete evacuation for over 3 months.

Classification. Bulk forming, osmotic, stimulant, stool softeners and lubricants, secretagogues (linaclotide), 5HT4 agonists (prucalopride) and peripherally acting opioid receptor antagonists.

• Bulk forming: ispaghula husk 3.5 to 7 g once or twice daily.
• Osmotic: lactulose 15 mL twice daily; macrogol 3350 with electrolytes, 1 to 3 sachets daily.
• Stimulant: senna 7.5 to 15 mg at night; bisacodyl 5 to 10 mg at night.
• Softener: docusate sodium 100 mg twice daily.
• Linaclotide 290 microgram once daily on an empty stomach.
• Prucalopride 2 mg once daily (1 mg in those over 65, with creatinine clearance under 30 mL/min or severe liver impairment).

Differences. Onset is 1 to 3 days for bulk and osmotic agents, 6 to 12 hours for oral stimulants and minutes to an hour for suppositories.

• Chronic constipation: fluids, fibre and mobility; then a bulk or osmotic agent; add a stimulant after 2 weeks if inadequate; then linaclotide or prucalopride.
• Opioid-induced constipation: start a laxative with the opioid; combine an osmotic agent with a stimulant; add a peripherally acting opioid receptor antagonist if still constipated.
• Faecal impaction: rectal measures first (glycerol or bisacodyl suppository, phosphate enema), then high-dose macrogol.
• Hepatic encephalopathy: lactulose titrated to 2 to 3 soft stools daily.
• Bowel preparation: macrogol or sodium picosulfate regimens.

Selection. Exclude obstruction, hypothyroidism, hypercalcaemia and constipating drugs (opioids, anticholinergics, calcium channel blockers, iron, antacids containing aluminium) before escalating.

Contraindications. Intestinal obstruction, ileus, toxic megacolon, acute abdomen, undiagnosed abdominal pain.

Adverse effects. Bloating, flatulence, cramps, diarrhoea, dehydration, hypokalaemia and hypomagnesaemia with prolonged stimulant or osmotic use, and hypermagnesaemia with magnesium salts in kidney impairment.

Interactions. Hypokalaemia from laxative abuse increases digoxin toxicity and QT risk. Bulk agents reduce absorption of digoxin, iron, lithium and carbamazepine.

Special groups.

• Pregnancy: bulk agents, lactulose, macrogol first; senna and bisacodyl short term.
• Breastfeeding: bulk, osmotic and low-dose senna acceptable.
• Children: weight-based paediatric products.
• Older adults: avoid prolonged stimulant use; check hydration.
• Kidney impairment: avoid magnesium and phosphate preparations.

Monitoring. Bowel frequency, potassium, magnesium and creatinine with prolonged use.

Referral and escalation. Rectal bleeding, weight loss, iron deficiency anaemia, change in bowel habit over 6 weeks in a person over 50, family history of colorectal cancer, a palpable mass, or constipation unresponsive to three agents needs investigation and specialist review.