Why Is My Poop Black? 7 Worrying Reasons to Watch For

Black stool is one of those symptoms that can mean almost nothing — or something that requires urgent medical attention. The problem is that it is not always easy to tell the difference from appearance alone. Getting that distinction right matters, because the serious causes of black stool include active upper gastrointestinal bleeding, which can become life-threatening if not treated promptly.

This article covers the seven most important reasons for black stool — both the worrying ones and the benign ones — along with clear guidance on when the colour of your stool should send you to a doctor the same day.

Q: Should I go to the emergency room if my stool is black? A: It depends on whether the black stool is tarry and foul-smelling (which strongly suggests digested blood) or simply dark (which may be caused by food or supplements). Black, tarry, sticky stool with a strong odour — called melena — is a medical emergency and warrants same-day evaluation at minimum. If you also have dizziness, fainting, rapid heartbeat, vomiting blood, or abdominal pain, go to the emergency room immediately. If the stool is simply dark and you recently ate black licorice, blueberries, or took iron supplements, you can monitor and reassess.

1. Upper Gastrointestinal Bleeding — The Most Urgent Cause

The most medically serious reason for black stool is bleeding from the upper gastrointestinal tract — the oesophagus, stomach, or the first part of the small intestine (duodenum). When blood from these areas travels through the digestive system, it is chemically altered by digestive acids and enzymes. The haemoglobin in the blood is broken down into haematin, which is black. The resulting stool is characteristically:

• Tarry — sticky and tar-like in consistency
• Shiny — often described as looking like black boot polish or tar
• Foul-smelling — distinctively different from normal stool odour, often described as sickly sweet or metallic
• Loose — often not formed in the way normal stools are

This specific type of black stool has a clinical name: melena. It is the definitive sign of significant upper GI bleeding and requires prompt medical evaluation without exception. The amount of blood needed to produce melena is approximately 50–100 millilitres — a meaningful bleed by any measure.

Upper GI bleeding can originate from peptic ulcers, oesophageal varices, Mallory-Weiss tears, gastric erosions, or tumours. It can present with melena alone — with no pain, no vomiting, no other immediately obvious symptoms — which is part of what makes it dangerous. A person can have a significant GI bleed with black stool as the only initial sign.

The distinction between melena and simply dark stool from food or supplements is critical. If you are unsure — if the stool is tarry, sticky, or unusually foul-smelling, or if you feel dizzy, faint, or weak alongside black stool — treat it as melena and seek medical evaluation the same day. The cost of a false alarm is far lower than the cost of delaying investigation of an active bleed.

2. Peptic Ulcers

Peptic ulcers — open sores that develop in the lining of the stomach or duodenum — are one of the most common causes of upper GI bleeding and, consequently, one of the most common medical reasons for black stool.

Ulcers develop when the protective mucous lining of the stomach or duodenum is eroded, typically by:

• Helicobacter pylori (H. pylori) infection — the most common cause worldwide, responsible for approximately 70–80% of duodenal ulcers
• Long-term use of NSAIDs — ibuprofen, aspirin, naproxen, and diclofenac reduce prostaglandin production, which weakens the protective stomach lining
• Stress ulcers — in critically ill patients, following major surgery, or severe burns
• Zollinger-Ellison syndrome — a rare condition involving acid-secreting tumours

When an ulcer bleeds into the stomach or duodenum, the blood mixes with gastric acid and begins producing the melena characteristic of upper GI bleeding. Peptic ulcers may produce black stool with or without the classic burning, gnawing upper abdominal pain — a significant number of ulcers, particularly in older adults and those taking NSAIDs regularly, are entirely painless until they bleed.

An H. pylori test (breath test, stool antigen test, or endoscopy-based biopsy) and appropriate antibiotic eradication therapy resolves most H. pylori-associated ulcers. NSAID-related ulcers require discontinuing or managing the causative medication alongside acid-suppressing treatment.

3. Oesophageal Varices and Portal Hypertension

Oesophageal varices are enlarged, fragile blood vessels in the wall of the oesophagus that develop as a consequence of portal hypertension — elevated blood pressure in the portal venous system, most commonly caused by cirrhosis of the liver.

When the liver is scarred and blood cannot flow through it normally, pressure builds in the portal system and blood is redirected through smaller vessels not designed to handle high-volume flow. The oesophageal veins dilate and become varices — thin-walled, high-pressure vessels that can rupture without warning and bleed massively.

Variceal bleeding is a medical emergency. It produces melena (and often haematemesis — vomiting blood that looks like coffee grounds or fresh blood), and it has a high mortality rate without rapid intervention. The populations most at risk are those with established liver disease: alcoholic cirrhosis, hepatitis B or C cirrhosis, and non-alcoholic fatty liver disease at advanced stages.

Black stool in a person with known liver disease — or with signs of liver disease such as jaundice, abdominal swelling, spider naevi, or a history of heavy alcohol use — should be treated as a potential variceal bleed until proven otherwise.

4. Gastritis, Erosion, and Medication-Related Damage

Gastritis — inflammation of the stomach lining — and the erosions it produces are a common source of low-level upper GI bleeding that can present as black stool without the acute severity of a perforated ulcer or variceal bleed.

Common causes of erosive gastritis and gastric damage that can produce black stool include:

Regular NSAID use: NSAIDs are the most common medication-related cause of gastric damage. They inhibit prostaglandin synthesis, reducing the protective mucous layer of the stomach and increasing vulnerability to acid damage. Black stool in someone taking ibuprofen or aspirin regularly — even in low-dose aspirin for cardiovascular protection — is a serious concern.

Alcohol: Heavy or chronic alcohol use is directly toxic to the gastric mucosa and causes erosive gastritis. Black stool in someone who drinks heavily warrants investigation for both gastric erosion and oesophageal varices.

Corticosteroids: Long-term steroid use increases the risk of gastric ulceration, particularly when combined with NSAIDs.

Anticoagulants and blood thinners: Warfarin, rivaroxaban, apixaban, and similar medications do not cause GI bleeding directly, but they prevent normal clotting — meaning minor mucosal damage that would normally heal quickly can bleed significantly.

If you are taking any of these medications and notice black stool, contact your GP or prescribing clinician promptly. Continuing the medication without investigation is not advisable.

5. Upper GI Cancers

Stomach cancer (gastric adenocarcinoma) and oesophageal cancer can both produce black stool through bleeding from the tumour site. These cancers are not the most common cause of black stool, but they are among the most important to identify — and early-stage cancers of both types may produce occult (hidden) bleeding with black or very dark stool as one of the early signs.

Features that make upper GI cancer worth investigating more urgently include:

• Black stool alongside unexplained weight loss
• Difficulty swallowing (dysphagia)
• Persistent indigestion or heartburn that is new or worsening
• Loss of appetite
• Feeling full very quickly after eating (early satiety)
• Anaemia on blood tests without an obvious explanation

These cancers are most common in adults over 55, in people with a family history of gastric cancer, in those with long-standing H. pylori infection, in smokers, and in those with Barrett’s oesophagus. The overlap between these risk factors and NSAID use and alcohol use is significant — which is why unexplained black stool in a middle-aged or older adult should always be investigated rather than assumed to be benign.

Upper GI cancers diagnosed at an early stage — before spread to lymph nodes or distant organs — have significantly better outcomes than those caught late. Black stool is one of the few early warning signs that these cancers may produce, and acting on it promptly rather than monitoring and hoping it resolves is the decision that makes a clinical difference.

6. Mallory-Weiss Tears and Mechanical Causes

A Mallory-Weiss tear is a longitudinal laceration in the mucosa at the junction of the oesophagus and stomach, typically caused by forceful or repeated vomiting, retching, or coughing. The tear bleeds into the GI tract and produces melena — or, if the bleed is heavier, fresh blood in vomit.

Mallory-Weiss tears are associated with:

• Episodes of excessive drinking followed by vomiting
• Severe morning sickness in pregnancy
• Bulimia-related purging
• Vigorous coughing

Most Mallory-Weiss tears stop bleeding spontaneously and heal without intervention. However, significant tears in patients with clotting abnormalities or liver disease can bleed heavily and require endoscopic treatment. Any black stool following an episode of forceful vomiting warrants medical evaluation.

Other mechanical causes of upper GI bleeding that can produce black stool include hiatus hernia with associated erosion, Dieulafoy’s lesion (an abnormally large submucosal artery that can bleed without ulceration), and radiation-induced damage from radiotherapy to the chest or upper abdomen.

7. Benign Causes — When Black Stool Is Not a Medical Emergency

Not all black stool has a medical cause. A number of foods, supplements, and medications produce entirely benign dark or black discolouration of stool without any bleeding involved. Knowing these allows you to distinguish a reassuring explanation from one that requires investigation.

Foods that can darken stool:

• Black licorice
• Blueberries and blackberries consumed in large amounts
• Dark grapes and grape juice
• Activated charcoal products
• Squid ink (used in some pasta and rice dishes)
• Black pudding and blood sausage

Supplements and medications:

• Iron supplements — one of the most common causes of black stool; the iron reacts with gut contents to produce a dark, sometimes greenish-black colour. Unlike melena, iron-related black stool is typically formed, not tarry, and does not have the distinctive foul smell of digested blood.
• Bismuth subsalicylate (found in Pepto-Bismol) — reacts with trace sulphur in saliva and the gut to produce bismuth sulphide, which is black. Black tongue can accompany black stool for the same reason.
• Activated charcoal — produces dramatically black stool, entirely harmless.

The distinguishing features that separate benign dark stool from concerning melena are consistency, smell, and context. Formed dark stool with a recent history of iron supplements or blueberry consumption is almost certainly benign. Tarry, sticky, foul-smelling black stool with no obvious dietary explanation requires medical evaluation.

Black stool that cannot be explained by diet or supplements is the kind of symptom that warrants same-day contact with a healthcare provider rather than watchful waiting. Knowing when to go to the emergency room versus when urgent care is the appropriate setting helps you act quickly and appropriately — because with GI bleeding, the speed of your response directly affects your outcome.

If you have been experiencing other digestive symptoms alongside black stool — including bloating, abdominal pain, or changes in bowel habits — the broader picture of what severe digestive symptoms can indicate is worth understanding as context for the conversation with your doctor.