Methylcellulose

Methylcellulose is a semi-synthetic cellulose ether used as a bulk-forming laxative. It absorbs water to form a lubricating gel and increases stool volume without being fermented, so it causes less gas than psyllium.

Classification. Bulk-forming laxative.

Mechanism. The swollen polymer expands stool mass, softens it and stimulates peristalsis. Because it is not fermented by colonic bacteria, it produces little gas.

Pharmacodynamics. Effect takes 12 to 72 hours, with full effect in a few days.

Pharmacokinetics. Not absorbed and not metabolised. Excreted unchanged in faeces.

Adult doses unless stated. Take with at least 240 mL of fluid.

Constipation.

• 1 to 2 g (two to four 500 mg tablets, or one heaped tablespoon of powder) up to three times daily.

• Maximum 6 g daily.

• An alternative labelled dose is 1.5 to 3 g twice daily with at least 300 mL of fluid.

• Effect is slow; allow 2 to 3 days before increasing the dose.

Faecal incontinence, ileostomy output and diverticular disease. 1 to 2 g once or twice daily to bulk and thicken stool.

Children. Age 6 to 11 years: half the adult dose. Not advised under 6 years.

Kidney and liver adjustment. None.

Contraindications. Intestinal obstruction; faecal impaction; colonic atony; difficulty swallowing or oesophageal stricture; hypersensitivity.

Common adverse effects. Bloating, abdominal fullness and mild cramp, usually settling in days.

Serious adverse effects.

• Oesophageal or intestinal obstruction if taken with too little fluid.

• Faecal impaction in dehydrated, immobile or opioid-treated patients.

• Rare hypersensitivity.

Major interactions. Separate other oral medicines by at least 2 hours.

• Digoxin, warfarin, iron, calcium, levothyroxine and carbamazepine: reduced absorption.

• Opioids and anticholinergics: slower transit and higher risk of impaction.

Special groups.

• Pregnancy: not absorbed; compatible.

• Breastfeeding: compatible.

• Children: half adult dose from 6 years.

• Older adults and bed-bound patients: ensure fluid intake; risk of impaction.

• Diabetes: no sugar content; no interference with glucose.

• Kidney impairment: no adjustment.

• Liver impairment: no adjustment.

Formulations. Tablets 500 mg; powder.

Administration. Take each dose with at least 240 mL of water. Chew tablets well before swallowing if the label advises. Do not take at bedtime without enough fluid.

Baseline. Cause of constipation; exclusion of obstruction; fluid intake.

Monitoring.

• Stool frequency, consistency and abdominal distension.

• Fluid intake.

• Reassess the need after 2 to 4 weeks.

Therapeutic drug monitoring. Not applicable.

Stopping. No taper is needed. Stop for distension, vomiting or no benefit after 2 to 4 weeks.

Referral and escalation. Seek urgent review for severe abdominal pain, vomiting or inability to pass flatus. Investigate red flags: weight loss, rectal bleeding, iron deficiency anaemia or change in bowel habit over age 50.