Diarrhoea lasting weeks: causes, red flags and when to get it checked

Chronic diarrhoea means the bowel is either secreting too much fluid, absorbing too little, moving too fast or being inflamed. Working out which depends on stool appearance (watery, fatty, bloody), timing (including at night), accompanying weight loss and what changed when it started.

• Irritable bowel syndrome with diarrhoea: urgency and loose stools with abdominal pain, usually no night-time diarrhoea or blood.

• Medicines: metformin, antibiotics, proton pump inhibitors, magnesium supplements and laxatives.

• Dietary triggers: sugar alcohols, caffeine, alcohol, lactose and excessive fibre.

• Coeliac disease: diarrhoea, bloating, weight loss, anaemia or mouth ulcers, even in adults.

• Persistent infection such as giardia or C. difficile after antibiotics, particularly after travel.

• Bile acid malabsorption, after gallbladder removal, with watery stools and urgency.

• Inflammatory bowel disease (ulcerative colitis, Crohn disease): blood, mucus, night diarrhoea, weight loss.

• Microscopic colitis, overactive thyroid, pancreatic insufficiency, and bowel cancer, which are less common.

Blood or mucus in stool, diarrhoea waking you at night, unintentional weight loss, fever, abdominal mass, anaemia, a family history of bowel cancer, inflammatory bowel disease or coeliac disease, a change in bowel habit over 50 and persistent symptoms after antibiotics or travel warrant investigation.

• Call 999 or go to A&E: severe dehydration with confusion or fainting, severe abdominal pain with distension, or heavy rectal bleeding.

• Same-day GP: bloody diarrhoea, high fever or signs of dehydration.

• Urgent GP referral: diarrhoea over six weeks with rectal bleeding, weight loss or anaemia, particularly over 50.

• GP within weeks: loose stools lasting over four weeks without red flags.

The doctor asks about duration, frequency, stool appearance, blood, night-time symptoms, weight loss, diet, alcohol, travel, antibiotic use, medicines and family history. They examine the abdomen, check for anaemia, thyroid signs and weight, and may do a rectal exam.

• Full blood count, ferritin, CRP, B12, folate, thyroid and coeliac blood tests.

• Faecal calprotectin: separates inflammatory bowel disease from IBS.

• FIT and stool cultures: for bowel cancer risk and infection, including C. difficile and giardia.

• Colonoscopy with biopsies: to look for colitis, microscopic colitis and tumours.

• Bile acid or breath tests, and pancreatic elastase in selected cases.

Drink plenty of fluids and use oral rehydration solution if dehydrated. Review caffeine, alcohol, sweeteners and recent medicine changes. Keep a food and stool diary. Do not use anti-diarrhoeal medicines such as loperamide for bloody or feverish diarrhoea without medical advice.

The NHS route is a GP with stool and blood tests then gastroenterology. Private gastroenterology offers direct access to colonoscopy and calprotectin testing.