Black tarry stool (melaena): causes, red flags and what to do

Melaena is distinct from ordinary dark stool. It is typically black like tar, glossy or sticky, difficult to flush away and has a strikingly unpleasant smell. The colour and smell arise because blood that has been exposed to stomach acid and gut bacteria for hours changes chemically before it reaches the toilet. By contrast, fresh red blood on the paper or in the bowl usually comes from the lower bowel or anus.

The NHS wording is worth knowing precisely. On its stomach ulcer page it tells people to call 999 or go to A&E if their poo has blood in it, or is black, sticky and very smelly. On its bowel cancer symptoms page it asks for an urgent GP appointment or NHS 111 contact if the poo is black or dark red or there is bloody diarrhoea. Both pages treat the symptom as a reason to be seen promptly rather than monitored for weeks.

Not every dark stool means bleeding. The NHS iron deficiency anaemia page lists darker than usual poo as a side effect of iron tablets, along with constipation, diarrhoea, tummy pain, heartburn and feeling sick. Iron-related dark stool is generally dark green-black or dark brown and formed, without the glossy, sticky consistency or the intense smell.

Other things that can darken stool include some foods and supplements, and medicines taken for the stomach. If you have recently started iron, think back to whether the change began after the first tablets. If you have not taken anything that explains it, or you have any of the accompanying features described below, assume it could be blood and seek urgent advice instead of trying to work out the cause yourself.

Sources of bleeding high in the digestive tract account for most true melaena. In the NHS description of stomach ulcers, the main causes of ulcers are infection with Helicobacter pylori bacteria and regular use of anti-inflammatory painkillers such as ibuprofen; ulcers are also more common in people who smoke and the risk increases with age. Bleeding from the stomach or intestine is listed among the complications of an untreated ulcer.

• Stomach or duodenal ulcer: often upper tummy pain that may burn and be worse at night, with indigestion, bloating and nausea; sometimes painless bleeding is the first sign.

• Anti-inflammatory painkillers and aspirin: can irritate the stomach lining; the NHS iron deficiency page names NSAIDs such as ibuprofen and aspirin among causes of bleeding in the stomach and intestines.

• Inflammation of the gullet or stomach lining: reflux, gastritis and erosions, which may follow alcohol or certain medicines.

• Cancers of the stomach or bowel: less common causes, but the NHS lists blood in the poo that may look red or black as a bowel cancer symptom, and an untreated H pylori infection raises the risk of stomach cancer.

• Enlarged veins in the gullet in people with advanced liver disease, and blood-thinning medicines that make any bleed heavier, which doctors consider when taking the history.

Black, tarry stool is itself a red flag. These are the NHS thresholds from its stomach ulcer and bowel cancer pages.

• Call 999 or go to A&E if your poo is black, sticky and very smelly, or has blood in it.

• Call 999 or go to A&E if you are vomiting bright red blood or your vomit looks like ground coffee.

• Call 999 or go to A&E if you have severe tummy pain, a tummy that hurts to touch, or chest pain that started suddenly.

• Call 999 or go to A&E if you are bleeding non-stop from your bottom, or there is a lot of blood or large clots when you poo.

• Faintness, a racing heart, cold clammy skin or collapse alongside black stool suggests significant blood loss and is an emergency.

• Ask for an urgent GP appointment or contact NHS 111 if your poo is black or dark red or you have bloody diarrhoea, and the NHS adds that stomach ulcer symptoms with loss of appetite, early fullness, unintentional weight loss, a lump in the tummy, difficulty swallowing, repeated vomiting or constant tiredness also need urgent advice.

• Do not drive yourself to A&E; ask someone else to drive, or call 999, and take your medicines with you.

The clinician will ask when the black stool began, how many times it has happened, whether you have vomited blood or coffee-ground material, and whether you have had tummy pain, heartburn, weight loss or trouble swallowing. They will ask about medicines (ibuprofen, aspirin, steroids, blood thinners), iron or other supplements, alcohol, smoking, previous ulcers, liver disease and family history of bowel or stomach cancer.

On examination they will check pulse, blood pressure and temperature, look for pallor and feel the tummy. A rectal examination can confirm black stool and exclude other sources. The NHS bowel cancer page notes that a doctor may feel your tummy and perform a rectal examination, and that you can ask for a male or female clinician or for someone else to be in the room.

Initial blood tests usually include a full blood count to look for anaemia, and tests of kidney and liver function. The NHS iron deficiency page notes the full blood count is the standard test for anaemia and that a faecal immunochemical test, which looks for blood in a poo sample, may be used when the cause is unclear.

The key test for suspected upper gut bleeding is gastroscopy, in which a thin flexible tube with a camera is passed through the mouth into the stomach. The NHS stomach ulcer page also lists a breath test for H pylori, a stool test and blood tests. Treatment depends on the cause: for ulcers the NHS describes antibiotics to treat H pylori and medicines such as proton pump inhibitors that reduce stomach acid, usually for between 1 week and 2 months. If bleeding is heavy, hospital care with fluids, sometimes blood transfusion, and an endoscopic procedure to stop the bleeding may be needed.

After an ulcer or bleed, the NHS suggests eating smaller meals, leaving 3 to 4 hours between your evening meal and bed, keeping to a healthy weight and managing stress. It advises not taking anti-inflammatory painkillers such as ibuprofen without speaking to a doctor first, avoiding smoking and keeping alcohol modest.

If you were found to be anaemic, iron treatment is usually continued for around 6 months according to the NHS, with repeat blood tests to confirm the level has recovered. Tell every clinician you see about the bleed, since it affects future decisions on painkillers and blood-thinning medicines.