Quick answer: The causes of black stools may be simple dietary factors – such as blood sausage, cranberries or medicines containing iron and bismuth – but they may also be caused by so-called melena, which results from gastric or duodenal ulcers, oesophageal varices or other upper gastrointestinal bleeding. As the symptoms of the two conditions are similar, a specialist examination is always required to determine the exact cause.
The causes of black stools can vary enormously, and it is often difficult to tell on your own whether it is due to a harmless dietary habit or a more serious health problem. Many people are alarmed when the colour of their stools suddenly changes, even though in most cases there is a simple explanation. However, dark, pitch-black, shiny and sticky stools – known medically as ‘melaena’ – can be a serious warning sign, as they may indicate the loss of at least 100 millilitres of blood from the digestive tract.
This article outlines the most common, harmless causes of black stools, before going on to describe the medical conditions where this symptom warrants serious attention. We also provide practical advice on when you should consult a specialist immediately, and what tests can help establish an accurate diagnosis.
What actually counts as black or dark stools?
Dark-coloured stools are not always the same. It is important to distinguish between what is commonly referred to as „darker” stools and what is medically known as ‘melaena’, which is a cause for concern.
Melaena is typically pitch-black, glossy, sticky or watery in consistency, and has a strong, characteristic odour. This type of discolouration indicates the presence of blood that has been broken down (digested) by gut bacteria, which most commonly originates from the upper gastrointestinal tract – that is, the oesophagus, stomach or duodenum. Less commonly, in cases of slower bowel transit, bleeding from lower sections of the digestive tract may also cause similar discolouration.
In contrast, stools that are simply a darker shade but otherwise of normal consistency are often attributable to a completely harmless cause. The key to distinguishing between them lies in a combined assessment of consistency, odour and accompanying symptoms.
Causes of black stools: foods and medicines that are harmless
Before anyone starts to panic, it’s worth thinking about what you’ve eaten or taken over the past few days. By far the most common causes of black stools are, in fact, everyday dietary and medication-related factors.
Foods that cause stools to turn dark
Eating certain foods can temporarily discolour your stools without posing any health risks. These include, amongst others:
- Blood sausage and blood-based dishes: the blood they contain, once digested, leaves a dark pigment in the stools.
- Blueberries and other dark berries: their natural pigments can easily alter the colour of the stools.
- Liquorice: the colouring agents it contains may also have a darkening effect.
- Dark, concentrated beetroot juice or a large quantity of red wine may also cause temporary discolouration.
These effects typically subside within a day or two, once the food in question has been eliminated from the body.
The effects of preparations containing iron and bismuth
Taking iron supplements is one of the most common non-haemorrhagic causes of dark or black stools. Iron reacts with gut bacteria to form dark-coloured compounds, which is a completely harmless side effect.
A similar mechanism underlies the effect of bismuth-containing medicines – such as the four-drug combination therapy used to treat Helicobacter pylori infection. Bismuth salts react with hydrogen sulphide in the large bowel, which directly discolours and darkens the stools. This phenomenon is a well-known and expected side effect and does not require any specific intervention.
When does black stool indicate a stomach haemorrhage?
If dietary and medication-related causes can be ruled out, black stools are most often caused by upper gastrointestinal bleeding. In such cases, the blood is digested by stomach acid and gut bacteria, which gives it its characteristic pitch-black colour.
The most common causes of upper gastrointestinal haemorrhage
There are numerous possible causes of upper gastrointestinal haemorrhage; in order of frequency, these are as follows:
- Gastric or duodenal ulcer (35–45%): the most common causes, in which Helicobacter pylori infection, antiplatelet therapy or long-term use of non-steroidal anti-inflammatory drugs (NSAIDs) may play a role.
- Oesophagitis (15–25%): it often causes hidden, i.e. occult, bleeding, which may also be triggered by reflux, drug-induced damage or infection.
- Erosive gastropathy and duodenopathy (collectively approx. 35%): lesions of the mucous membrane which bleed when they reach the depth of an ulcer.
- Oesophageal and gastric varices: complications of portal hypertension resulting from liver cirrhosis, which can lead to severe, even life-threatening, bleeding.
- Mallory–Weiss syndrome: a tear in the mucous membrane at the junction of the stomach and oesophagus, which typically occurs following severe vomiting.
- Less common causes: neoplastic lesions, angiodysplasia and various vascular abnormalities in the mucosa.
What other symptoms might accompany the bleeding?
As well as black stools, it is worth looking out for the following accompanying symptoms, which may confirm a suspicion of gastric haemorrhage:
- Dizziness, weakness or a feeling of faintness
- Pallor, fatigue (signs of anaemia)
- Abdominal pain or a burning sensation in the stomach area
- Vomiting, possibly containing blood or coffee-ground-like material
- Rapid heartbeat or low blood pressure
If these symptoms occur together, urgent medical attention is required.
When should we seek medical advice immediately?
It is not always necessary to panic, but certain symptoms clearly call for immediate medical attention. Black stools accompanied by persistent, coffee-ground-like vomiting, abdominal pain, dizziness or loss of consciousness require emergency treatment.
It is important to note that in the event of sudden, unexplained discolouration lasting longer than a few days – even in the absence of any accompanying symptoms – it is advisable to seek a specialist’s opinion. Advanced age, taking anticoagulants or anti-inflammatory medication, and a history of peptic ulcer disease are all factors that increase the risk and warrant a more rapid investigation.
How is the underlying cause of black stools diagnosed?
In order to make an accurate diagnosis, the specialist begins the examination by taking a detailed medical history, followed by a physical examination. Afterwards, an endoscopic examination is carried out if necessary.
If upper gastrointestinal bleeding is suspected, the gastroscopy (gastroscopy) It provides an accurate picture of the condition of the oesophagus, stomach and duodenum, and also allows for the immediate treatment of any source of bleeding. If there is any suspicion that the lower gastrointestinal tract is also affected, a colonoscopy may also be necessary. If both examinations are warranted, they can be carried out in a single session, with a single preparation and a single anaesthetic – this significantly reduces the burden on the patient.
It is important to make it clear that, for professional reasons, during endoscopic examinations, only anaesthesia They use general anaesthesia, not sedation. During the anaesthesia, the patient sleeps deeply and pain-free, whilst their vital signs are continuously monitored by an anaesthetist, ensuring that the procedure is safe and leaves no memory of it.
How does Endomedix fit into all this?
Endomedix’s team of gastroenterology specialists is dedicated to cases where the cause of black or dark stools needs to be investigated. Through a detailed medical history and targeted questions, the specialists first assess whether the cause is dietary or medication-related, or whether there may be a more serious problem with the digestive tract.
Where symptoms warrant it, Endomedix offers gastroscopy and colonoscopy under general anaesthesia, or separate gastroscopy and colonoscopy procedures, using modern, high-resolution endoscopic equipment and carbon dioxide insufflation, which minimises post-procedure discomfort. Any abnormalities found – such as polyps or bleeding ulcers – can be treated painlessly by the specialist during the examination itself.
If you notice that your stools are black or unusually dark and you’re unsure what might be causing it, please book an appointment for a specialist gastroenterology consultation or directly gastroscopy and colonoscopy under general anaesthesia. It is during the consultation with a specialist that a decision is made as to whether further, targeted tests are required.
Don’t put off getting it checked out – the causes of black stools can be identified quickly
In most cases, the causes of black stools can be easily explained by dietary habits or medication, but the symptom can sometimes indicate a more serious condition, one that may even require urgent medical attention. Rather than speculating on your own, the safest course of action is always to see a specialist, who can quickly and accurately determine the underlying cause.
Don’t wait until your symptoms get worse. The team of specialists at Endomedix is on hand to provide you with reassuring answers and, if necessary, to start treatment straight away.
Frequently asked questions
How long can black stools caused by iron last?
Whilst taking iron supplements, your stools may remain dark, but they will return to their normal colour within a few days of finishing the treatment.
Is it dangerous if my stools have only been dark once?
A single occurrence in itself rarely poses a serious problem, particularly if you have recently eaten dark-coloured food or taken an iron supplement. However, if the problem recurs or you experience any accompanying symptoms, it is advisable to consult a specialist.
How quickly should you see a doctor if you have any worrying symptoms?
If black stools are accompanied by dizziness, vomiting, abdominal pain or fainting, immediate medical attention is required. In the case of asymptomatic but persistent discolouration, it is advisable to consult a specialist within a few days.
Is a gastroscopy or colonoscopy painful?
At Endomedix, examinations are carried out under general anaesthesia, so the patient feels no pain or discomfort whatsoever during the procedure.
How much does the examination cost?
The fees for specialist gastroenterology consultations and endoscopic examinations vary from centre to centre and can be claimed through a health insurance fund – Endomedix staff can provide information on the exact prices.
