Diarrhoea: Causes, Home Care, and When to See a GP

What Is Diarrhoea?

The World Health Organization definition, quoted by NICE CKS, is the passage of three or more loose or liquid stools per day, or more often than is normal for you. NICE CKS itself describes diarrhoea more simply as the abnormal passing of loose or liquid stools with increased frequency, increased volume, or both. On the Bristol Stool Chart, types 6 and 7 represent diarrhoea: mushy with ragged edges (type 6) or entirely liquid with no solid pieces (type 7). Both indicate faster-than-normal colonic transit, meaning stool has not had enough time for adequate water reabsorption.

Acute Causes of Diarrhoea

• Norovirus (winter vomiting bug)
  Most common UK cause. Extremely contagious. Self-limiting in 2-3 days. Strict handwashing, avoid food preparation for 48h after recovery.
• Campylobacter
  UK's most common bacterial food poisoning. Undercooked poultry. Can be severe, may require antibiotics.
• Salmonella
  Eggs, poultry, reptiles. Self-limiting in 4-7 days for most. GP if severe or persistent.
• E. coli (STEC, including O157)
  Undercooked beef, unpasteurised milk, contaminated water. STEC strains can cause haemolytic uraemic syndrome - seek urgent care.
• Post-antibiotic diarrhoea
  Antibiotics disrupt gut microbiome. Usually mild and self-resolving; if severe or associated with fever, check for C. difficile.
• Traveller's diarrhoea
  Contaminated water or food abroad. Usually E. coli strains. ORS + short-course azithromycin if severe.
• Food intolerance (acute exposure)
  Large lactose or fructose load, or accidental gluten exposure in coeliac disease.

Chronic Causes of Diarrhoea

• IBS-D
  The most common chronic diarrhoea cause in otherwise healthy adults. No structural abnormality. Rome IV diagnosis requires 6 months history, >25% stools types 6-7.
• Bile acid diarrhoea (BAD)
  Affects ~1% of UK population, up to 35% of IBS-D patients. Often missed. Common after cholecystectomy. Treatable with colestyramine or colesevelam.
• Coeliac disease
  Autoimmune gluten response causing villous atrophy. Prevalence ~1% UK. Blood test (tTG-IgA) then endoscopic biopsy. Strict lifelong gluten-free diet.
• Inflammatory bowel disease (Crohn's/UC)
  Structural inflammation. Usually has blood or mucus. Urgent investigation required.
• Microscopic colitis
  Collagenous or lymphocytic colitis. Watery diarrhoea (type 7), often in older women. Normal colonoscopy; biopsy required for diagnosis.
• Metformin
  Up to 20-30% of metformin users experience diarrhoea. Slow titration and extended-release formulation reduces this.

Probiotics for Diarrhoea: What the Evidence Shows

Saccharomyces boulardii CNCM I-745 (available as Florastor) has the strongest clinical evidence for diarrhoea of any probiotic strain. It is particularly effective for post-antibiotic diarrhoea (including C. difficile prevention), traveller's diarrhoea, and acute infectious diarrhoea. A Cochrane meta-analysis found it significantly reduced the duration of acute diarrhoea in adults and children.

For IBS-D specifically, Bifidobacterium longum subsp. longum 35624 (sold as Alflorex, and long known in the literature as B. infantis 35624) has UK-based clinical trial data. NICE CG61 names no strain and no brand; it advises anyone who chooses to try a probiotic to take it for at least four weeks at the manufacturer's dose while monitoring the effect. See our partner site probioticvsprebiotic.com for a full evidence comparison by strain.