The essentials

Black stool is often harmless: iron tablets, bismuth stomach remedies, black licorice or blueberries can all darken it. Stool that is jet black, sticky like tar and unusually foul-smelling is different: it can be digested blood from the stomach or upper intestine and needs a medical check the same day. Blood tests (hemoglobin, BUN) and an upper endoscopy are what usually settle the question, not a home stool kit.

When is black stool an emergency?

Blood from high in the digestive tract turns black as it is digested[1], and acute upper bleeding carries a reported death rate of 2 to 10%[7]. Use this scale to decide how fast to act.

When to see a doctor

  • Emergency: call 911 if black or tarry stool comes with fainting, dizziness when you stand, a racing heart, cold or clammy skin, confusion, shortness of breath, or vomiting blood or material that looks like coffee grounds. These can signal heavy bleeding or shock[2].
  • Today, even if you feel well if the stool is truly black, sticky or tar-like and you cannot link it to iron, bismuth or food, especially if you take aspirin, an anti-inflammatory painkiller or a blood thinner, have had an ulcer, or have liver disease[1].
  • At your next appointment if dark stool started with a new iron supplement or bismuth remedy, has a normal texture, and you have no other symptoms.

What causes black stool that is not dangerous?

Most dark stools come from something swallowed in the last day or two and keep a normal shape and smell.

  • Common Iron supplements. Iron tablets often make stool very dark green to black. Unlike blood, oral iron does not turn a guaiac stool test positive in volunteers taking it[9].
  • Common Bismuth subsalicylate (the pink stomach remedy). It can darken both stool and tongue; this change is harmless and usually clears a few days after the last dose[11].
  • Possible Dark foods and charcoal. Black licorice, blueberries, blood sausage and activated charcoal can all darken stool[1].

For every other shade, see our guide to what each stool color means.

What serious conditions cause black, tarry stool (melena)?

Doctors call black, sticky, foul-smelling stool melena. It usually means bleeding in the esophagus, stomach or duodenum, the first part of the small intestine[1]. When a patient reports melena, an upper source becomes much more likely (likelihood ratio 5.1 to 5.9)[6].

  • Common Peptic ulcer. Ulcers of the stomach or duodenum are the most common cause of acute upper bleeding[1]. Blood thinners and antiplatelet drugs such as aspirin raise the risk of bleeding[1], and anti-inflammatory painkillers such as ibuprofen or naproxen are a classic cause of ulcers. See peptic ulcer symptoms and causes.
  • Common Gastritis and erosions, an inflamed or raw stomach lining[1]. See gastritis and its lab tests.
  • Possible Varices, swollen veins in the esophagus or stomach, most often caused by liver cirrhosis[1].
  • Possible Mallory-Weiss tear, a tear in the lower esophagus after forceful vomiting[1].
  • Rare Cancer of the esophagus, stomach or duodenum, or abnormal blood vessels in the upper gut[1]. See stomach cancer warning signs.

Red blood or clots point lower, toward the colon[6]. See visible blood in stool.

What tests will the doctor order for black stool?

Is this blood, and how much has been lost? Blood tests show severity and anemia; endoscopy finds and often treats the source[3].

Tests commonly used to investigate black stool
TestWhat it answersLearn more
Complete blood count, hemoglobinHow much blood has been lost; whether anemia is presentHemoglobin results, complete blood count
BUN and creatinineWhether digested blood is being absorbed high in the gutBUN/creatinine ratio, blood urea nitrogen
Ferritin and iron panelWhether bleeding has been slow and long-standingFerritin levels, iron panel
Fecal occult blood (guaiac or FIT)Hidden blood in a stool sample; mainly built for colon screeningOccult blood test
H. pylori stool antigenInfection that causes many ulcersH. pylori antigen
Upper endoscopyViews the esophagus, stomach and duodenum; can stop bleedingDone by a gastroenterologist[3]
Colonoscopy, capsule endoscopyUsed when the upper endoscopy finds nothingCapsule views the small intestine[3]

Have a stool or blood report after an episode of dark stool? BloodSense walks you through every line and helps you prepare questions for your doctor.

Get my stool test explained

How do I read the results of these tests?

Hemoglobin: a normal value early on is not reassuring

Right after a bleed, hemoglobin can still look normal because the remaining blood has not yet been diluted, so doctors repeat it. A hemoglobin below 8 g/dL makes a severe bleed needing urgent treatment more likely[6], and hospital teams usually transfuse below 7 g/dL[4].

BUN out of proportion to creatinine points upward

Digested blood is absorbed as protein, raising blood urea nitrogen while creatinine stays steady. A BUN-to-creatinine ratio above 30 makes an upper source clearly more likely (likelihood ratio 7.5)[6].

Low ferritin tells a longer story

Low ferritin, often with small red cells, suggests slow blood loss over weeks or months[2]. See our guide to anemia.

A negative FIT does not rule out an upper bleed

The fecal immunochemical test (FIT) detects the globin part of hemoglobin, which is broken down during digestion. In a Mayo Clinic study, an immunochemical test caught 1 of 42 upper bleeding lesions, against 11 of 42 for a guaiac test[8]. Guaiac results can be skewed by red meat, vitamin C above 250 mg a day and some painkillers[10].

The risk score that decides hospital or home

Emergency teams combine blood results such as BUN and hemoglobin with blood pressure, pulse and a few questions into the Glasgow-Blatchford score. US guidelines suggest that people scoring 0 or 1 can often go home with prompt outpatient follow-up[4]; UK guidance uses the score at first assessment and considers early discharge at 0[5].

What should I note before seeing the doctor?

These are the questions you will likely be asked[1].

  • When the dark stool started, and how many bowel movements have been dark
  • Texture and smell: formed and ordinary, or sticky, shiny and tar-like
  • Iron tablets or bismuth remedies taken this week
  • Aspirin, ibuprofen, naproxen, clopidogrel, warfarin or any other blood thinner
  • Dark foods eaten in the last two days
  • Past ulcers, liver disease, heavy drinking, forceful vomiting

Questions to ask your doctor

  1. Do you think my stool color comes from blood, or from my iron or other medicines?
  2. Should my hemoglobin be checked again in the next few days?
  3. Do I need an upper endoscopy, and how soon?
  4. Is it safe to keep taking my aspirin, painkillers or blood thinner for now?

Frequently asked questions

How long does stool stay dark after stopping bismuth?

The darkening from bismuth subsalicylate usually fades within a few days of the last dose, and the tongue clears too[11].

Can I use a home FIT kit to find out whether my black stool is blood?

No. FIT is designed to find blood from the colon. Blood from the stomach is partly digested before it reaches the stool, and FIT often misses it[8].

My stool is very dark green, not truly black. Does that count?

Check it against white toilet paper. Very dark green with a normal texture is typical of iron tablets. Jet black, shiny and sticky is the pattern that suggests melena[1].

Should I stop my blood thinner if my stool turns black?

Not on your own: these drugs often protect against stroke or heart attack. Call the prescriber. After treatment of a bleed, outcomes appear better when they are restarted early under supervision[7].

References

  1. MedlinePlus Medical Encyclopedia. Black or tarry stools. medlineplus.gov (accessed October 21, 2026)
  2. National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms & causes of GI bleeding. niddk.nih.gov (accessed October 21, 2026)
  3. National Institute of Diabetes and Digestive and Kidney Diseases. Diagnosis of GI bleeding. niddk.nih.gov (accessed October 21, 2026)
  4. Laine L, Barkun AN, Saltzman JR, Martel M, Leontiadis GI. ACG clinical guideline: upper gastrointestinal and ulcer bleeding. Am J Gastroenterol. 2021;116(5):899-917. doi:10.14309/ajg.0000000000001245 (accessed October 21, 2026)
  5. National Institute for Health and Care Excellence. Acute upper gastrointestinal bleeding in over 16s: management (CG141). Recommendations 1.1.1-1.1.2. nice.org.uk (accessed October 21, 2026)
  6. Srygley FD, Gerardo CJ, Tran T, Fisher DA. Does this patient have a severe upper gastrointestinal bleed? JAMA. 2012;307(10):1072-9. doi:10.1001/jama.2012.253 (accessed October 21, 2026)
  7. Stanley AJ, Laine L. Management of acute upper gastrointestinal bleeding. BMJ. 2019;364:l536. doi:10.1136/bmj.l536 (accessed October 21, 2026)
  8. Harewood GC, McConnell JP, Harrington JJ, Mahoney DW, Ahlquist DA. Detection of occult upper gastrointestinal tract bleeding: performance differences in fecal occult blood tests. Mayo Clin Proc. 2002;77(1):23-8. doi:10.4065/77.1.23 (accessed October 21, 2026)
  9. McDonnell WM, Ryan JA, Seeger DM, Elta GH. Effect of iron on the guaiac reaction. Gastroenterology. 1989;96(1):74-8. doi:10.1016/0016-5085(89)90766-x (accessed October 21, 2026)
  10. MedlinePlus Lab Tests. Fecal occult blood test (FOBT). medlineplus.gov (accessed October 21, 2026)
  11. MedlinePlus Drug Information. Bismuth subsalicylate. medlineplus.gov (accessed October 21, 2026)
  12. HealthCare.gov. Preventive care benefits for adults. healthcare.gov (accessed October 21, 2026)

This article is general health information. It does not replace a medical examination, diagnosis or treatment. If your stool is black and tarry, or you feel faint, seek care without waiting.