Functional Diarrhoea

Functional diarrhoea is a functional bowel disorder defined by recurrent loose or watery stools (more than 25 percent of stools mushy or entirely liquid) without predominant abdominal pain or bothersome bloating, present for at least 3 months with onset at least 6 months earlier.

• It is a diagnosis of exclusion after organic causes have been reasonably ruled out. If abdominal pain is a prominent feature, the correct label is irritable bowel syndrome with diarrhoea.
• Mechanisms: accelerated colonic transit, bile acid malabsorption (about 25 to 30 percent of patients labelled functional diarrhoea), visceral hypersensitivity, altered intestinal permeability, post-infectious change, dietary triggers (fructose, sorbitol, caffeine, alcohol), altered microbiota.
• Prevalence about 5 percent of adults; slightly more common in men and younger adults than IBS.
• Prognosis: chronic but non-progressive; no increased mortality.

• Painless or minimal discomfort loose stools, often morning, often after meals (gastrocolic reflex), urgency, occasionally faecal incontinence, no nocturnal diarrhoea.
• No weight loss, bleeding, fever or anaemia.
• Red flags: nocturnal diarrhoea, weight loss, rectal bleeding, iron deficiency anaemia, age over 50 with new onset, family history of colorectal cancer, coeliac disease or inflammatory bowel disease, steatorrhoea (floating, pale, offensive stools), palpable abdominal or rectal mass, raised inflammatory markers, recent antibiotics, travel, immunosuppression.
• History: drug review (metformin, proton pump inhibitors, magnesium and laxatives, statins, antibiotics, selective serotonin reuptake inhibitors, NSAIDs, colchicine, orlistat), artificial sweeteners, alcohol, caffeine, prior cholecystectomy or terminal ileal disease.
• Examination: BMI, anaemia, thyroid, abdominal palpation, digital rectal examination (anal tone, impaction).

• Initial: full blood count, CRP or ESR, ferritin, vitamin B12 and folate, urea and electrolytes, liver function, thyroid function, HbA1c, coeliac serology (tissue transglutaminase IgA with total IgA), faecal calprotectin (below 100 micrograms/g makes inflammatory bowel disease unlikely; 100 to 250 repeat; above 250 refer), stool culture and ova, cysts and parasites, Clostridioides difficile toxin if recent antibiotics, Giardia antigen, faecal elastase if steatorrhoea.
• Colonoscopy with biopsies (to exclude microscopic colitis) if age over 50 with new symptoms, red flags, or persistent unexplained diarrhoea.
• Bile acid malabsorption: SeHCAT scan (7 day retention under 15 percent abnormal; under 10 percent moderate; under 5 percent severe) where available, or serum 7-alpha-hydroxy-4-cholesten-3-one (C4) above 52.5 ng/mL, or a short therapeutic course of a bile acid sequestrant.
• Hydrogen breath testing if suspected lactose, fructose or sorbitol intolerance or small bowel bacterial overgrowth.
• Differential: IBS-D, microscopic colitis, bile acid diarrhoea, coeliac disease, giardiasis, thyrotoxicosis, carcinoid, VIPoma, pancreatic exocrine insufficiency, drug induced.

• Explanation and reassurance. Dietary modification: reduce caffeine, alcohol, sorbitol, xylitol and fructose; regular meals; adequate fluids; low FODMAP diet for 4 to 6 weeks under dietitian guidance with structured reintroduction; reduce insoluble fibre; consider soluble fibre such as ispaghula husk 3.5 g sachet once or twice daily in water.
• Loperamide 2 mg orally after each loose stool (initial 4 mg then 2 mg after each loose stool; usual maximum 16 mg per day, or 8 mg per day for self-medication), or prophylactically 1 to 2 mg 30 minutes before meals or social events; titrate to the lowest effective dose; long term low doses of 2 mg once or twice daily are acceptable. Contraindicated in active colitis, bloody diarrhoea, C. difficile, abdominal distension or ileus. Reports of cardiac toxicity (QT prolongation, torsades) at very high doses (over 16 mg per day); avoid taking with drugs that inhibit CYP3A4/P-glycoprotein.
• Bile acid sequestrants for confirmed or presumed bile acid diarrhoea:
  • Colestyramine 4 g orally once daily with a meal, titrated to 4 g up to three or four times daily (maximum 36 g per day); mix the sachet in 150 mL fluid; give other drugs at least 1 hour before or 4 to 6 hours after colestyramine; side effects include bloating, constipation, fat soluble vitamin deficiency with long term use (check vitamins A, D, E, K).
  • Colesevelam 625 mg tablets, 1.875 g orally once or twice daily with meals (up to 3.75 g per day).
  • Review efficacy at 4 weeks; if no response in 6 to 8 weeks, stop.
• Antispasmodics are usually not needed. Low dose amitriptyline 10 mg orally at night, increased by 10 mg every 1 to 2 weeks to 30 to 50 mg, has benefit in refractory cases (monitor for anticholinergic effects, sedation, QTc).
• Eluxadoline 100 mg orally twice daily with food (75 mg in those intolerant; contraindicated in absent gallbladder, alcohol misuse, pancreatitis, severe hepatic impairment) is licensed in IBS-D but not in functional diarrhoea.
• Probiotics: use a single strain for 4 weeks; stop if no benefit.
• Follow up in 4 to 8 weeks. Refer if alarm features, treatment failure, or diagnostic uncertainty.