1. What does blood in the stool mean?
Blood in the stool is a symptom, not a condition in its own right. The causes range from harmless haemorrhoids through anal fissures to inflammatory bowel diseases, stomach ulcers or bowel cancer. Decisive for the assessment are the colour, the amount and accompanying symptoms such as pain, fever or weight loss.
A distinction is made between visible blood (bright red, dark red or black) and occult blood — that is only detectable via a stool test (FOBT, iFOBT) and is part of bowel cancer screening.
Important: even though the most common cause (haemorrhoids) is harmless, every first occurrence of bleeding and every bleeding from the age of 45 to 50 should be assessed by a doctor — even a small bleed can point to a polyp or an early tumour.
2. Colour and origin: bright red, dark or tarry stool?
The colour gives a first clue as to where the bleeding probably lies:
Bright red blood (fresh blood, haematochezia)
Usually comes from the lower digestive tract — rectum, anus, sigmoid colon. Most common causes: haemorrhoids, anal fissures, polyps, diverticula. Typical: blood on the toilet paper or coatings on the stool.
Dark red or chestnut-brown blood
Points to a source of bleeding further up in the large bowel — e.g. diverticula, tumours or inflammatory bowel diseases. The blood is usually mixed with the stool.
Black, sticky tarry stool (melaena)
Arises from blood from the upper digestive tract (stomach, duodenum, upper small bowel). Stomach acid breaks down the blood and discolours it black. Tarry stool smells characteristically foul-sweetish. Always have it assessed by a doctor — typical causes are stomach ulcers, oesophageal tears or, more rarely, tumours.
Important: black stool can also have harmless causes — iron supplements, activated charcoal, blueberries or beetroot also colour the stool dark. Real tarry stool is sticky, shiny and smells strong. If unsure, always have it assessed by a doctor.
Occult blood (invisible)
Is only recognised via a stool test (iFOBT) and can point to polyps or early tumours — even if the stool looks normal. This is why regular bowel cancer screening is so important.
3. Common causes in the lower digestive tract
Haemorrhoids
By far the most common cause of bright red blood on the toilet paper. Haemorrhoids are enlarged vascular cushions in the anal canal — they can bleed, itch or protrude during a bowel movement. Favoured by chronic constipation, sedentary activity, pregnancy and strong straining.
Anal fissure
A small tear in the anal mucosa — typical are severe pain during and after a bowel movement as well as bright red blood on the paper. Often a consequence of hard stool or chronic constipation.
Diverticula and diverticulitis
Outpouchings of the large bowel wall, especially in the sigmoid colon. They can bleed (usually painlessly, dark red) or become inflamed — then with fever and severe pain in the lower left abdomen. Common in people over 60.
Polyps and bowel cancer
Polyps are benign growths of the bowel mucosa that can, however, become malignant. They usually bleed weakly and unnoticed — hence the occult blood. Bowel cancer (colorectal carcinoma) is the second most common cancer in Germany and usually arises from such polyps over many years. Early signs: occult blood, changed bowel habits, weight loss, anaemia.
Inflammatory bowel disease (IBD)
Crohn's disease and ulcerative colitis often cause bloody diarrhoea, mucus coatings, abdominal pain and weight loss. First manifestation often in young adulthood (20–40 years).
Infectious colitis
Bacterial bowel infections (Campylobacter, Shigella, EHEC, Salmonella) can trigger bloody diarrhoea — typical after travelling or spoiled food. See also diarrhoea.
4. Causes in the upper digestive tract
Stomach and duodenal ulcers
Ulcers of the gastric or duodenal mucosa can bleed — typically with tarry stool, sometimes also with vomiting of blood. Main causes: Helicobacter pylori infection and long-term intake of NSAIDs (ibuprofen, diclofenac, acetylsalicylic acid). More on this under heartburn.
Reflux oesophagitis and Mallory-Weiss syndrome
Inflammation of the oesophagus or mucosal tears after severe vomiting can lead to bleeding — usually bright red blood when vomiting, in the stool as melaena.
Oesophageal varices
Enlarged veins in the oesophagus with advanced liver cirrhosis. A variceal bleed is a life-threatening emergency — always call 112 (in the US: 911) immediately.
Tumours of the upper digestive tract
Stomach cancer, oesophageal cancer and, more rarely, small bowel tumours can bleed — usually chronically and occultly with iron deficiency anaemia as the first sign.
Emergency: heavy bleeding, circulatory symptoms (dizziness, racing heart), fever, severe abdominal pain — go to the emergency department immediately or call 112
