Bulk Forming Laxatives

Bulk forming laxatives are soluble or insoluble fibre products that absorb water in the gut, expand stool mass and stimulate peristalsis through distension. They are the gentlest laxatives and the first choice in mild constipation, and are also used to bulk stool in irritable bowel syndrome and in some diarrhoeas.

Pharmacokinetics. Not absorbed. Effect appears after 2 to 3 days of regular use.

• Ispaghula husk (psyllium) 3.5 g sachet, 1 to 2 sachets once or twice daily in at least 150 mL of water.
• Sterculia 1 to 2 sachets once or twice daily after meals.
• Methylcellulose; use the product label for dosing.
• Dietary fibre target 25 to 30 g daily from food.

Differences. Ispaghula has the best evidence in chronic constipation and irritable bowel syndrome. Methylcellulose causes less gas.

• Mild chronic constipation and hard stool.
• Irritable bowel syndrome with constipation or mixed symptoms.
• Haemorrhoids and anal fissure, to soften stool.
• Mild diarrhoea, to bulk liquid stool.

Selection. Start low and increase over 2 weeks to reduce bloating. Ineffective in opioid-induced constipation and slow transit constipation.

Contraindications. Dysphagia, oesophageal stricture, intestinal obstruction, faecal impaction, megacolon, hypersensitivity (anaphylaxis to psyllium is described).

Adverse effects. Bloating, flatulence, abdominal discomfort. Oesophageal or bowel obstruction when swallowed dry or taken with too little fluid.

Interactions. Reduced absorption of digoxin, iron, lithium, carbamazepine and warfarin. Take other drugs at least 2 hours before or after.

Special groups.

• Pregnancy: first choice.
• Breastfeeding: acceptable.
• Children: reduced doses from the product label.
• Older adults and immobile patients: ensure adequate fluid or avoid.
• Kidney impairment and fluid restriction: avoid unless fluid intake is adequate.

Monitoring. Stool frequency and consistency; swallowing and abdominal distension.

Referral and escalation. Persistent symptoms after 4 weeks at full dose, abdominal pain with vomiting, or rectal bleeding.