Diarrhoea: Causes, Tests & When to See a Doctor

What Is Diarrhoea?

Diarrhoea is loose, watery stools occurring more than three times a day. Most cases are acute (lasting a few days) and caused by infection; chronic diarrhoea lasting weeks needs further investigation.

Common Causes

• Viral infections (norovirus, rotavirus)
• Bacterial infections (E. coli, Salmonella)
• Food intolerances (lactose, gluten)
• Medications, especially antibiotics
• Irritable bowel syndrome (IBS)
• Inflammatory bowel disease (Crohn's, ulcerative colitis)
• Stress and anxiety

Tests Your Doctor May Order

Managing Diarrhoea at Home

• Drink oral rehydration solution or water with electrolytes
• Eat bland foods (bananas, rice, toast) as tolerated
• Avoid dairy, caffeine and fatty foods until recovered
• Avoid anti-diarrheal medication if fever or blood is present

Frequently Asked Questions

Acute or chronic, and why the line is drawn at four weeks

Diarrhoea lasting under two weeks is nearly always infective and self-limiting, and investigation beyond checking hydration is rarely needed. Between two and four weeks it is called persistent, and a stool test becomes reasonable. Beyond four weeks it is chronic, and the likely causes change entirely: coeliac disease, inflammatory bowel disease, microscopic colitis, bile acid malabsorption, thyroid overactivity, pancreatic insufficiency, and medication side effects all become plausible, and irritable bowel syndrome is diagnosed only once the others have been considered.

Features that make a simple explanation less likely

• Blood or mucus in the stool, which is never a feature of irritable bowel syndrome and needs investigation.
• Waking at night to open the bowels, which distinguishes organic disease from functional causes fairly reliably.
• Unintentional weight loss, fever, or anaemia found on a blood test.
• Onset after age 50, or a family history of bowel cancer or inflammatory bowel disease.
• Pale, greasy stools that float and are hard to flush, suggesting fat malabsorption.

Medication and dietary causes that are easily missed

Metformin is a very common cause and often improves on the modified-release preparation. Magnesium-containing antacids and supplements, proton pump inhibitors, SSRIs, colchicine and many antibiotics all cause diarrhoea. Antibiotic-associated diarrhoea occasionally reflects Clostridioides difficile, which should be considered when it is severe, bloody, or follows a recent course of antibiotics or a hospital stay. On the dietary side, sugar alcohols such as sorbitol and xylitol in sugar-free gum and sweets are a recurring and easily overlooked culprit, as is a sudden increase in caffeine or fibre.

Rehydration and when to stop taking loperamide

Oral rehydration salts remain the mainstay for acute diarrhoea, because the glucose they contain drives sodium and therefore water absorption in a way plain water cannot. Loperamide is reasonable for short-term symptom control in uncomplicated cases, but should be avoided if there is fever or blood in the stool, where slowing the bowel can be harmful. Seek assessment if you cannot maintain fluids, pass very little urine, become confused or lightheaded on standing, or if symptoms persist beyond a week.