Why Is My Poop Black? Causes, Melena, and When to Worry

Black stool falls into two very different categories. The first is cosmetic: iron pills, Pepto-Bismol, a bag of licorice, or a heavy serving of blueberries changed the color and nothing is wrong. The second is melena, a distinct black-and-tarry stool with a foul, sticky odor caused by digested blood from somewhere in the upper gastrointestinal tract. One needs no action. The other is a medical situation that gets worse the longer it waits.

The difference between the two is usually obvious once you know what to look for. This piece walks through every common cause, the specific texture and smell clues that separate benign from dangerous, and the numbers gastroenterologists use to decide when to escalate.

What Actually Turns Stool Black

Two chemistries. In the benign case, iron or bismuth reacts with hydrogen sulfide produced by gut bacteria to form iron sulfide or bismuth sulfide, both of which are black. That color coats the stool but doesn't change its texture, and it comes and goes with the supplement or medication.

In melena, the black is from digested blood. When blood spills into the stomach or upper small intestine, gastric acid and gut enzymes oxidize the iron in hemoglobin to hematin, a dark pigment. Bacteria in the small bowel and colon then break the blood down further as it moves through, producing the sticky, tarry consistency and a characteristic sweet-metallic-putrid smell that gastroenterologists learn to recognize on sight. A common clinical rule of thumb: it takes about 50 mL of blood pooled in the stomach to darken stool that far, and the blood has to sit long enough (usually eight hours or more) to fully oxidize (Wilson, Clinical Methods, 3rd edition, NCBI).

That eight-hour window matters. Bleeding that moves through the gut too fast to be digested comes out red or maroon (hematochezia), not black. So melena implies bleeding that is either upper GI or slow enough for full transit-time oxidation. About 90 percent of clinically important melena traces to a source above the ligament of Treitz, the anatomical hinge between the duodenum and jejunum (Wilson, NCBI Clinical Methods).

The Fast Test: Is It Melena or Not

Before combing through causes, run this triage. Melena has three features together:

• Color. Deep black. Not dark brown, not brownish-black. The comparison used most often is fresh tar or shoe polish.
• Texture. Sticky, shiny, and often loose. It clings to the bowl and to toilet paper in a way ordinary stool doesn't. If it looks matte and holds a normal Bristol form, it is almost certainly not melena.
• Odor. Unmistakably foul, distinct from normal stool smell. Clinicians describe it as sweet-metallic or putrid. If you have to ask whether it smells worse than usual, it probably isn't melena.

All three features together, especially in someone with reflux, ulcer history, chronic NSAID use, alcohol use, or new lightheadedness, is a hospital-level concern. Any one feature alone, especially without the smell, is much more likely to be a supplement or food.

Cause #1: Iron Supplements

Iron is the single most common reason otherwise healthy adults see black stool. About 5 to 10 percent of the iron in a typical supplement gets absorbed; the rest passes into the colon, where the ferrous iron binds hydrogen sulfide from gut bacteria to form iron sulfide. That's the black.

The rate is dose-dependent and formulation-dependent. In a randomized comparison of iron preparations in pregnant patients, black stool showed up in 8 percent of women on ferrous bisglycinate 25 mg, 22 percent on ferrous fumarate 40 mg, and 31 percent on ferrous sulfate 50 mg (Milman et al., 2024, PMC). If you started iron in the last week and now see black stool, this is your answer.

Iron-induced black stool is matte, not tarry, holds Bristol form 3 or 4, and does not carry the melena smell. It also stops when you stop the supplement. Prenatal vitamins produce the same effect because they typically contain 27 mg or more of iron.

One warning that matters: iron can mask early melena because both are black. If you're on iron and something changes - stool becomes tarry, sticky, or reeks - do not assume iron. Melena on top of iron is well documented, and physicians will not fault you for coming in to sort it out.

Cause #2: Bismuth (Pepto-Bismol, Kaopectate)

Bismuth subsalicylate, the active ingredient in Pepto-Bismol and Kaopectate, is one of the most predictable stool-color changers in the pharmacy. The bismuth ion combines with trace sulfur in the mouth and gut to form bismuth sulfide, a jet-black compound. The tongue often blackens first because the reaction starts on contact with oral bacteria (Drugs.com clinical Q&A on bismuth).

Both the tongue discoloration and the stool color are harmless and reverse within a few days of stopping the medication. Bismuth is also a common cause of what looks like melena but isn't, so it is worth knowing if you or someone you're helping just treated a bout of traveler's diarrhea or heartburn with pink liquid.

A note on the salicylate side: bismuth subsalicylate contains an aspirin-like compound, and in people already at risk (chronic NSAID users, kidney disease, kids with viral illness), it isn't risk-free. This isn't about stool color; it's a reason not to lean on it heavily.

Cause #3: Foods That Turn Stool Dark

Several foods carry enough dark pigment to shift stool color, especially when eaten in volume:

• Black licorice (real licorice with glycyrrhizin, not just black candy). A large serving reliably darkens stool.
• Blueberries. A pint in a smoothie can push stool close to black. The anthocyanin pigments are stable through digestion.
• Blood sausage, black pudding, and morcilla. These contain cooked blood, which behaves in the gut similarly to digested blood: the iron oxidizes and the stool turns dark. This is the food most likely to be confused with melena.
• Beets in large amounts. Usually reddish, but concentrated servings can look near-black.
• Oreos, dark chocolate, and heavily dyed foods. Occasional cause when eaten in quantity, especially in kids.

Dietary black stool resolves in one to three bowel movements and matches the timing of the food. Cleveland Clinic's diet-and-color primer covers the full list (Cleveland Clinic Health Essentials).

Cause #4: Peptic Ulcers - The Big One

When melena is real, peptic ulcer disease is by far the most likely source. Peptic ulcers account for 28 to 59 percent of upper GI bleeds across large observational series (BLUE study, Scandinavian Journal of Gastroenterology, 2020). Two forces drive almost every case:

• H. pylori infection. A gram-negative bacterium that damages the stomach lining's protective mucus and lets acid erode it. H. pylori is behind more than 90 percent of duodenal ulcers and roughly 70 to 80 percent of gastric ulcers in populations where it's endemic. Test-and-treat with a course of antibiotics and a proton pump inhibitor resolves the ulcer diathesis in most patients.
• NSAIDs. Ibuprofen, naproxen, aspirin, and diclofenac inhibit the prostaglandins that normally protect the stomach lining. Chronic use raises upper GI bleeding risk four- to six-fold, and about one in five people on daily nonaspirin NSAIDs develops a demonstrable ulcer on endoscopy (Lanas et al., PubMed 17926519).

Combining the two (NSAIDs on top of H. pylori) is worse than either alone. So is layering aspirin or NSAIDs on top of an anticoagulant like warfarin, apixaban, or clopidogrel. Older adults, people with a prior ulcer, and heavy drinkers sit at the top of the risk stack.

Peptic ulcer bleeding classically presents as painless melena, sometimes with vomiting of coffee-ground material (partially digested blood). Frank abdominal pain is not required; many bleeding ulcers don't hurt, which is why silent presentations catch people off guard.

Cause #5: Esophageal Varices

Varices are dilated veins in the lower esophagus that develop when portal pressure rises, usually from cirrhosis. They aren't rare in the population at risk: 30 percent of patients with cirrhosis have varices at diagnosis, and prevalence rises to 90 percent over 10 years. Once large varices are present, the annual risk of first bleed is about 15 percent, and the mortality of a first variceal bleed sits near 20 percent (Meseeha & Attia, StatPearls / NCBI).

Variceal bleeding is dramatic in a way ulcer bleeding often isn't: vomiting bright red blood is common, and melena can be accompanied by rapid lightheadedness or fainting. Anyone with known liver disease, chronic hepatitis C, alcohol use disorder, or a family history of cirrhosis who sees black stool should treat it as an emergency without waiting to see if it repeats.

Cause #6: Gastritis, Mallory-Weiss Tears, and Other Upper GI Sources

Beyond peptic ulcers and varices, several less common upper GI sources also produce melena:

• Erosive gastritis. Inflammation of the stomach lining from alcohol, NSAIDs, severe stress, or H. pylori. Can bleed diffusely rather than from one ulcer crater.
• Esophagitis. Severe reflux, especially with a hiatal hernia, can erode the lower esophagus and bleed.
• Mallory-Weiss tears. Longitudinal splits at the gastroesophageal junction caused by forceful retching or vomiting - a night of heavy drinking, hyperemesis in pregnancy, food poisoning. Often present with bright red blood in vomit and melena a day later.
• Dieulafoy lesion. A rare abnormal submucosal artery that can bleed briskly. Usually diagnosed at endoscopy.
• Upper GI malignancy. Stomach cancer and, less commonly, small bowel tumors. More often causes chronic slow bleeding and iron-deficiency anemia than dramatic melena, but it does happen.
• Post-procedure bleeding. After endoscopy with biopsy or polyp removal, small amounts of blood can produce transient melena.

Slow bleeding in the small intestine - from angiodysplasia, Meckel's diverticulum, or Crohn's disease - can also cause melena if transit is slow enough for the blood to be fully digested (Kim et al., Gastrointestinal Bleeding, StatPearls / NCBI). Our Crohn's disease symptoms guide covers when to consider inflammatory bowel disease as a background contributor.

Cause #7: Activated Charcoal and Other Meds

A few more entries on the medication side worth knowing:

• Activated charcoal. Used in poisoning treatment and in some digestive supplements. Turns stool black uniformly and predictably for 24 to 48 hours after use. Harmless.
• Some antibiotics (particularly those that shift the microbiome hard) can occasionally darken stool without causing bleeding.
• Anticoagulants (warfarin, apixaban, rivaroxaban, dabigatran) don't turn stool black on their own but sharply raise the risk that a small ulcer or erosion will bleed. Melena on an anticoagulant is a same-day evaluation.
• Chronic aspirin, even low-dose baby aspirin, raises bleeding risk over time. In the elderly it is a common silent contributor.

Black Specks vs Black Stool

Small black specks are a different phenomenon and usually not concerning. They can come from undigested seeds (chia, poppy, blackberry pips), spice fragments (pepper, cumin), banana pith, or leafy greens broken down partway. If the specks are moving through with otherwise normal-colored, normal-formed stool, they're almost always food fiber.

Specks that look shiny or tarry and appear alongside dark stool are different and should be treated the way you would treat melena. Same for anything that resembles coffee grounds.

Black Stool in Kids and Babies

Meconium, a newborn's first stool in the days after birth, is normally black or very dark green and sticky. That's expected and not melena. After the first week, stool should shift toward yellow (breastfed) or tan-brown (formula-fed). A black stool in an older infant or child is worth a pediatrician call, especially if the child is on iron drops (common cause), has been vomiting, or looks pale or tired.

Kids can also get melena, though rarely. Causes to know: swallowed maternal blood in breastfed newborns (usually cracked nipples), Meckel's diverticulum, and Henoch-Schonlein purpura in older kids. None of these are safe to watch at home.

When Black Stool Is an Emergency

Call 911 or go to an emergency department for black stool with any of the following:

• Vomiting blood, or vomiting material that looks like coffee grounds
• Lightheadedness, fainting, or a racing heart
• Cold, clammy, or pale skin
• Chest pain or shortness of breath
• Confusion or new severe weakness
• Known cirrhosis, esophageal varices, or a prior GI bleed
• Anticoagulant use (warfarin, apixaban, rivaroxaban, dabigatran, clopidogrel)
• Severe abdominal pain

These are signs of significant blood loss or an active bleed that can lose enough volume quickly to cause shock. Waiting to see if the next stool looks better is not the right move.

For black stool that lacks those features but isn't clearly iron, bismuth, or food, contact a doctor within 24 hours. The NIDDK's guidance on GI bleeding is that any unexplained melena warrants prompt evaluation, typically starting with a stool guaiac or fecal immunochemical test, blood count, and referral for endoscopy if positive (Kamboj et al., Upper Gastrointestinal Bleeding, StatPearls / NCBI).

What the Workup Looks Like

If you present with confirmed melena, the standard workup follows a predictable sequence. Blood pressure and heart rate go first - the earliest sign of significant blood loss is not pallor but a rise in resting heart rate. Blood is drawn for a complete blood count (to check for anemia), a type and screen (in case transfusion is needed), coagulation studies, and liver panel. Two IV lines go in.

An upper endoscopy (EGD) is the diagnostic gold standard. It identifies the bleeding source in more than 90 percent of cases and can treat most of them in the same session - clipping a bleeding vessel, injecting epinephrine, banding varices, or applying thermal coagulation (StatPearls / NCBI on Upper GI Bleeding). Most patients also start a proton pump inhibitor IV drip immediately, which reduces re-bleeding rates for ulcers.

H. pylori testing is routine for anyone with an ulcer. Treatment is a two-week combination of antibiotics and acid suppression that clears the infection in about 90 percent of cases and cuts recurrent ulcer risk substantially.

What to Do Right Now

Practical, in order:

1. Check the last 48 hours. Iron pills, a prenatal vitamin, Pepto-Bismol, Kaopectate, activated charcoal, a large serving of blueberries or black licorice, or blood sausage. If the answer is yes and you feel fine, this is almost certainly the cause. Watch for one to three more bowel movements.
2. Assess the stool. Matte and Bristol form 3 or 4 with no unusual smell tips benign. Shiny, sticky, loose, foul-smelling tips melena.
3. Check yourself. Any lightheadedness on standing, unusual fatigue, palpitations, or new pale complexion? Those are more important than the stool alone.
4. Note your risk factors. Recent NSAID use, alcohol use, known ulcer or reflux, liver disease, anticoagulants. All raise the bar for calling a doctor early.
5. If in doubt, escalate. Black stool that isn't clearly benign should be evaluated. If you're on iron and something has changed about the color, texture, or smell, don't assume iron.
6. Track it. A single ambiguous stool is anecdote. Dating the change, noting what medications and foods you took, and photographing the pattern is what a gastroenterologist will actually use.

Color changes are one of the more useful signals a bowel movement can give you, but only if you're catching them accurately. Our stool color chart covers the full spectrum, and the Bristol Stool Chart guide puts form and transit time together so you can read both at once. For the other end of the spectrum, our blood in stool guide covers the bright red case.

What the Science Doesn't Say

A few loose claims worth pushing back on. Black stool is not a sign of "detoxing" or a "cleanse working." Activated charcoal turning stool black is charcoal in the stool, not toxins leaving the body; the same charcoal in a hospital setting is used to bind ingested poisons and is not a wellness intervention. And melena is not diagnostic on its own for any single condition. It's a location signal (upper GI, or slow lower GI transit) that narrows the workup, not a specific diagnosis. The specific diagnosis is what endoscopy is for.