Bristol Stool Chart; classes of laxatives and their modes of action

The Bristol Stool Form Scale — purpose and diagram. A validated seven-point pictorial scale developed at Bristol Royal Infirmary that classifies stool by form, which correlates with whole-gut transit time. It gives an objective, reproducible way for patients and nurses to describe stool, and is used to diagnose and monitor constipation, diarrhoea and faecal incontinence.

Clinical use of the chart. Types 1–2 indicate constipation, 3–4 are normal (Type 4 ideal), 5 suggests inadequate fibre, and 6–7 indicate diarrhoea. It is used to satisfy the Rome IV criterion of "lumpy or hard stools", to titrate laxatives (target Type 3–4), to monitor bowel preparation, to distinguish true diarrhoea from overflow around faecal impaction, and as a nursing chart in institutional and palliative care.

Bulk-forming laxatives. Ispaghula/psyllium, methylcellulose and sterculia. Mode of action — they are non-absorbable hydrophilic polysaccharides that increase faecal mass and retain water, distending the colon and stimulating peristalsis. Onset 2–3 days; they require an adequate fluid intake. Avoid in immobile, dehydrated, opioid-treated or impacted patients because of the risk of obstruction and faecal loading.

Osmotic laxatives. Polyethylene glycol (macrogol) — first-line, with the best evidence, and lactulose, a non-absorbable disaccharide that is fermented by colonic bacteria to organic acids, drawing water osmotically (and useful in hepatic encephalopathy). Magnesium salts and sodium phosphate act more rapidly. Cautions: avoid magnesium salts in renal impairment (hypermagnesaemia) and phosphate enemas in the frail elderly (hyperphosphataemia, hypocalcaemia and acute kidney injury); lactulose causes bloating and flatulence.

Stimulant (irritant) laxatives. Senna, bisacodyl and sodium picosulfate. Mode of action — they stimulate the myenteric plexus to increase colonic motility and also reduce water and electrolyte absorption. Onset 6–12 hours orally (bisacodyl suppository within 1 hour). They cause abdominal cramps and hypokalaemia with prolonged use, and anthraquinones produce melanosis coli (benign and reversible). They are the most useful class alongside opioids.

Softeners, lubricants and newer agents. Docusate sodium is a surfactant that lowers stool surface tension but is largely ineffective and no longer recommended; liquid paraffin must be avoided in the elderly because of lipoid aspiration pneumonia; glycerine suppositories act as rectal lubricants and stimulants. Newer classes: secretagogues — lubiprostone (a chloride channel-2 activator) and linaclotide/plecanatide (guanylate cyclase-C agonists) which increase intestinal chloride and water secretion; the prokinetic prucalopride, a selective 5-HT 4 agonist that accelerates colonic transit; and PAMORAs — methylnaltrexone, naloxegol and naldemedine, peripherally acting mu-opioid receptor antagonists that reverse opioid-induced constipation without reversing analgesia.

AGA/ACG joint guideline on chronic idiopathic constipation (2023), reinforced by the WGO Global Guideline 2025 — polyethylene glycol and senna carry strong recommendations; linaclotide, plecanatide, prucalopride and lubiprostone are conditional recommendations for refractory cases; there is an explicit recommendation against docusate and other stool softeners; and PAMORAs are the agents of choice for opioid-induced constipation, in which bulk-forming laxatives should be avoided altogether. Cross-reference: The full stepwise management of constipation, including faecal impaction, is given in Question 5; this answer covers the chart and the pharmacology of each laxative class.