What Does Black Stool Look Like and When to Worry

Black stool can range from completely harmless to a genuine medical emergency, and the difference often comes down to appearance, consistency, and smell. True melena, the medical term for black stool caused by bleeding in the upper digestive tract, has a distinctive tarry, sticky texture and an unusually foul odor that sets it apart from the dark stools you might see after taking an iron supplement or eating a bowl of blueberries. Understanding what to look for can help you decide whether your next step is simply reviewing what you ate yesterday or getting to a doctor quickly.

What Melena Actually Looks Like

The word “black stool” covers a wide range of appearances, and not all of them signal trouble. Melena specifically refers to black, tarry stools caused by bleeding in the upper gastrointestinal tract, which includes the esophagus, stomach, and the first section of the small intestine called the duodenum.¹ What makes melena look the way it does is the chemical transformation blood undergoes as it passes through your digestive system. Hemoglobin in the blood gets broken down by stomach acid and digestive enzymes, converting it into a compound called hematin, which is jet black. By the time it exits, it no longer looks like blood at all.

The texture is the most telling feature. Melena stools tend to be sticky, almost like tar or thick motor oil. They cling to the toilet bowl and are difficult to flush away. The color is uniformly dark, often described as coal-black or pitch-black rather than just dark brown. And the smell is distinctly offensive in a way that goes beyond normal stool odor. People who have experienced it or who work in healthcare often describe it as unmistakable once you have encountered it. If your stool is simply dark brown or has dark specks in it, that is not the same thing.

In some cases, slow bleeding from the ascending colon, part of the lower digestive tract, can also produce melena rather than the bright red or maroon blood you might expect from lower GI bleeding.¹ The key factor is transit time: the longer blood sits in the digestive system, the darker it becomes. A fast bleed in the lower intestine tends to come out red or maroon, which is called hematochezia, while a slow bleed anywhere has more time to darken.

Harmless Reasons Your Stool Might Be Black

Before you panic, it is worth considering whether something you swallowed is responsible. Several everyday substances can turn your stool black without any bleeding at all, and recognizing these saves a lot of unnecessary anxiety.

Iron supplements: One of the most common culprits. Unabsorbed iron turns stool black, and the effect can be striking. This happens because the dose exceeds the intestine’s ability to absorb all of the iron, so the excess passes through and oxidizes.² Higher doses produce the effect more reliably, and side effects like nausea and changes in bowel habits also increase with dose.³
Bismuth subsalicylate: The active ingredient in Pepto-Bismol and similar products reacts with trace amounts of sulfur in your saliva and digestive tract to form bismuth sulfide, a black compound. This can darken both your stool and your tongue. The effect is temporary and harmless.
Certain foods: Blueberries, black licorice, dark chocolate in large quantities, beets (which can also cause red stools that mimic bleeding), and foods with dark food coloring can all darken stool noticeably.
Activated charcoal: Sometimes taken as a supplement or used in medical settings for certain types of poisoning, charcoal predictably turns stool very dark or black.

The stool produced by these substances typically differs from melena in important ways. Iron-darkened stool, for instance, is usually dark green-black rather than the pure pitch-black of melena, and it does not have the same tarry, sticky consistency. It also lacks that distinctively foul smell. If you recently started an iron supplement or took Pepto-Bismol, the simplest test is to stop the substance for a couple of days and see if the color normalizes. If it does, you have your answer.

When Black Stool Is a Medical Emergency

The real concern with black stool is upper gastrointestinal bleeding, which can range from a slow ooze to a life-threatening hemorrhage. Several features should prompt you to seek medical attention quickly rather than waiting to see what happens.

The most urgent scenario is melena accompanied by other symptoms of significant blood loss. If you are feeling lightheaded, dizzy, or faint, if your heart is racing, if you are unusually pale, or if you feel weak or confused, these suggest enough bleeding to affect your circulation. Vomiting blood, whether it looks bright red or has a dark, coffee-ground appearance, alongside black stool points strongly toward active upper GI bleeding. This combination of hematemesis and melena occurs in a substantial proportion of people with upper GI bleeds.⁴

Even without those dramatic warning signs, true melena on its own warrants medical evaluation. You do not need to be hemorrhaging for the bleeding to matter. A slow, persistent bleed can lead to significant anemia over days or weeks, and the underlying cause, whether it is an ulcer, erosion, or something else, needs to be identified.

A reasonable rule of thumb: if you cannot identify a clear dietary or supplement explanation for your black stool, or if stopping a suspected cause does not resolve it within a day or two, see a doctor. If you have symptoms of blood loss alongside the black stool, do not wait at all.

Common Causes of Upper GI Bleeding

When melena does turn out to be caused by bleeding, the source is usually something in the stomach or duodenum. In both adults and children, duodenal and gastric ulcers are the most frequent findings. One study examining children who underwent upper endoscopy for melena found that duodenal ulcers were the most common cause by a wide margin, followed by esophageal varices, inflammation of the esophagus, gastric ulcers, and gastritis.⁵ The pattern in adults is broadly similar, with peptic ulcer disease dominating as the single most common cause.

Esophageal varices, which are swollen veins in the esophagus typically caused by liver disease, are another significant source. These bleeds can be severe and tend to occur in people with known cirrhosis or heavy alcohol use. Tears in the lining of the stomach or esophagus, sometimes called Mallory-Weiss tears, are another cause. These often follow forceful vomiting or retching. Tumors in the stomach or upper intestine can also bleed, though they represent a smaller fraction of cases overall.

Medications That Increase the Risk

Certain medications make upper GI bleeding more likely, and they are among the most widely used drugs in the world. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, naproxen, and aspirin damage the protective lining of the stomach, which is why doctors often recommend taking them with food or using the lowest effective dose for the shortest period.

A systematic review using a precise method to measure actual blood loss in stool found that most NSAIDs and low-dose aspirin increased daily fecal blood loss from about half a milliliter per day at baseline to roughly one to two milliliters per day. That sounds small, but aspirin at full anti-inflammatory doses pushed the average to about five milliliters daily. And the averages conceal individual variation: about five percent of people taking NSAIDs lost five milliliters or more per day, and one percent lost ten milliliters or more. For people taking high-dose aspirin, nearly a third had daily losses of five milliliters or more.⁶

Blood thinners like warfarin and the newer direct oral anticoagulants do not directly damage the stomach lining the way NSAIDs do, but they make any existing bleed harder to stop and more likely to become clinically significant. If you take a blood thinner and notice black, tarry stools, that combination deserves prompt medical attention. Corticosteroids used long-term can also increase ulcer risk, particularly when combined with NSAIDs.

How Doctors Investigate Black Stool

When you show up with melena, the first priority is figuring out how stable you are. Doctors assess blood pressure, heart rate, and signs of significant blood loss before anything else. If the bleeding appears serious, intravenous fluids and sometimes blood transfusions come first, with diagnostic procedures following once you are stabilized.⁷

The primary diagnostic tool is an upper endoscopy, where a flexible camera is passed through the mouth into the esophagus, stomach, and duodenum. This allows the doctor to see the source of bleeding directly and, in many cases, treat it at the same time. Bleeding ulcers can be injected with medication, cauterized, or clipped shut. Bleeding varices can be banded. In a study of children with melena who had endoscopy within 24 hours, the bleeding source was identified in the vast majority of cases.⁵ Treatment for peptic ulcers typically combines endoscopic intervention with acid-suppressing medication, while variceal bleeding is treated with banding and drugs that reduce blood flow to the varices.⁸

When upper endoscopy does not find the source, the bleeding may be coming from further down the digestive tract. A newer approach involves swallowing a capsule-sized camera that photographs the entire intestine as it passes through. In one study of patients with melena and a negative upper endoscopy, this capsule approach identified a source of bleeding in about five out of six cases, with the small bowel and colon being roughly equal contributors.⁹

Fecal Occult Blood Tests and Their Limits

You may have heard of at-home stool tests that detect hidden blood, sometimes called fecal occult blood tests. These are widely used in colorectal cancer screening programs and work well in that context. However, using them to evaluate visible black stool is a different matter, and their reliability drops significantly when used as a diagnostic test for symptomatic patients.

One major issue is false positives. A chart review found that the vast majority of patients who were tested were taking one or more medications capable of causing a false-positive result, and diet modification before testing was rare.¹⁰ Iron supplements, NSAIDs, red meat, and certain vegetables can all trigger a positive result on older guaiac-based tests without any actual bleeding present. Newer immunochemical tests are more specific to human blood but still do not tell you where the bleeding is coming from.

If your stool is visibly black and tarry, a fecal occult blood test does not add much. You already know something is going on. The next step is seeing a doctor who can evaluate you properly, not running a screening test designed for people without symptoms. The test has its place, but confirming what your eyes are already telling you is not it.

Black Stool in Newborns and Infants

Parents of newborns often encounter an alarming sight in the first day or two of life: very dark, almost black stool. This is meconium, the thick, greenish-black substance that fills a baby’s intestines before birth. It is entirely normal, made up of amniotic fluid, mucus, and other materials the baby ingested in the womb. Meconium typically transitions to a lighter, seedy stool within a few days as breastfeeding or formula feeding gets established.

More concerning is when a newborn passes what appears to be bloody black stool. One recognized cause is swallowed maternal blood, either during delivery or from cracked nipples during breastfeeding. This condition, sometimes called benign melena neonatorum, can present as a surprisingly large amount of dark or bloody stool very shortly after birth.¹¹ In breastfed infants, checking for nipple fissures is an important step when dark stools appear.¹² A simple lab test called the Apt test can distinguish maternal blood from the baby’s own blood, which helps doctors quickly determine whether further investigation is needed.

That said, black stool in a newborn that cannot be explained by meconium timing or swallowed maternal blood does require medical evaluation. Rare but serious causes include bleeding disorders and anatomical problems in the GI tract. If vomiting and black stools appear before breastfeeding has even started, swallowed blood during delivery is a strong possibility, but the baby still needs to be assessed.¹²

Older Adults and Higher Stakes

Upper GI bleeding carries more risk as people age, for several overlapping reasons. Older adults are more likely to take NSAIDs for arthritis or daily aspirin for heart protection. They are more likely to be on blood thinners. They are more likely to have liver disease, ulcer-prone conditions, or cancers of the digestive tract. And their bodies are less resilient to blood loss: a bleed that a healthy thirty-year-old could compensate for may cause dangerous drops in blood pressure or trigger a cardiac event in someone in their seventies.

Studies comparing older and younger patients with upper GI bleeding show that melena is the presenting symptom in roughly similar proportions across age groups, around seventy percent of cases.⁴ But outcomes tend to be worse in older patients because of the higher burden of other health conditions and the medications they take. If you are over sixty and notice black, tarry stools, treat it with more urgency rather than less.

The Color Spectrum From Red to Black

One thing that confuses people is the overlap between different types of GI bleeding and the colors they produce. It helps to think of stool color as a rough indicator of where and how fast the bleeding is happening.

Bright red blood on the stool or toilet paper usually points to something near the exit: hemorrhoids, anal fissures, or rectal lesions. Maroon or dark red stool, called hematochezia, typically comes from the lower GI tract, such as the colon.¹ Black, tarry stool points to the upper GI tract. But these are generalizations, not absolute rules. A very brisk upper GI bleed can move through so quickly that it comes out red or maroon rather than black, and a slow lower GI bleed can darken into something that resembles melena.

The context matters as much as the color. Someone with known hemorrhoids who sees a streak of red on the toilet paper after straining is in a very different situation from someone who suddenly fills the bowl with dark red stool and feels faint. Both involve blood, but the urgency is not remotely comparable.

Alcohol, Liver Disease, and Variceal Bleeding

Among the more dangerous causes of melena is bleeding from esophageal varices, which are engorged veins that develop in the esophagus when the liver is scarred and blood cannot flow through it normally. This condition, called portal hypertension, is most commonly caused by cirrhosis from chronic alcohol use or viral hepatitis. When these fragile veins rupture, the bleeding can be massive and rapid.

Variceal bleeding accounts for a meaningful portion of upper GI bleeds and has a high mortality rate compared to ulcer-related bleeding. It requires specialized treatment: endoscopic banding to physically ligate the bleeding vessels, combined with medications that reduce pressure in the portal vein system.⁸ People with known liver disease who develop melena should treat it as an emergency, because the volume of blood loss can escalate quickly.

Even without a prior diagnosis of liver disease, heavy drinkers who notice black stool should not dismiss it. Alcohol itself irritates the stomach lining and can cause gastritis or ulcers, and chronic heavy use raises the risk of varices whether or not the person has been formally told they have cirrhosis. The liver can be substantially damaged before symptoms become obvious in other ways.

What You Can Do Before Seeing a Doctor

If you notice black stool and are trying to figure out your next move, a few practical steps can help you and your doctor sort things out more efficiently. First, think through what you have eaten and what supplements or medications you have taken in the past 48 hours. Iron supplements, bismuth-containing antacids, and dark-colored foods are the most common innocent explanations. If any of these apply, try stopping them for a day or two and observing whether the color returns to normal.

Second, pay attention to associated symptoms. Fatigue, lightheadedness, unusually rapid heartbeat, abdominal pain, nausea, or vomiting, especially vomiting that looks like coffee grounds, all point toward something more serious than a dietary cause. Even without those symptoms, true tarry stool with an unusual smell warrants a call to your doctor.

Third, if you are on blood thinners or take NSAIDs regularly, mention this upfront when you seek care. These medications change the clinical picture and may influence how aggressively the doctor investigates. Do not stop a prescribed blood thinner on your own, but make sure your medical team knows about it quickly so they can make decisions about continuing or pausing it.

1
Osmosis from Elsevier. Melena · What Is It, Causes, Symptoms, Bleeding, Treatment, and More
2
PubMed Central. Low-Dose Prophylactic Oral Iron Supplementation (Ferrous Fumarate, Ferrous Bisglycinate, and Ferrous Sulphate) in Pregnancy Is Not Associated With Clinically Significant Gastrointestinal Complaints: Results From Two Randomized Studies – Section: 3.4. Frequency of Black Stools
3
The American Journal of Medicine. Are we giving too much iron? Low-dose iron therapy is effective in octogenarians
4
PubMed Central. Acute Upper Gastrointestinal Bleeding in Hexagenerians or Older (≥60 Years) Versus Younger (<60 Years) Patients: Clinico-Endoscopic Profile and Outcome
5
PubMed Central. Causes of Melena and Effective Examination Strategies in Children
6
PubMed Central. Faecal blood loss with aspirin, nonsteroidal anti-inflammatory drugs and cyclo-oxygenase-2 selective inhibitors: systematic review of randomized trials using autologous chromium-labelled erythrocytes – Section: RESULTS
7
PubMed. Upper Gastrointestinal Bleeding: Etiologies and Management
8
PubMed Central. Diagnosis and Management of Upper Gastrointestinal Bleeding
9
European Journal of Gastroenterology & Hepatology. A new panenteric capsule endoscopy-based strategy in patients with melena and a negative upper gastrointestinal endoscopy: a prospective feasibility study
10
PubMed Central. Fecal occult blood testing as a diagnostic test in symptomatic patients is not useful: a retrospective chart review
11
PubMed Central. Copious amount of bloody stool in a newborn shortly after birth
12
Journal of Biosciences and Medicines. Etiology and Treatment Advances of Hematochezia in Infants Aged ≤3 Months – Section: 2. Medical Causes of Early Infant Hematochezia