Medical Oncologist
CION Cancer Clinics
Black, tarry stools: what melena means and when to act | CION Cancer Clinics
Black, tarry stool is rarely a sign of colon cancer. Most black stool comes from iron tablets or dark foods, and true tarry stool, called melena, usually means bleeding from the stomach or food pipe, often from an ulcer. Because it can mean active bleeding, sticky black stool you cannot explain needs a doctor today. This page explains the common causes, the warning signs and the tests that find the source. At CION Cancer Clinics, our specialists help you judge which bowel symptoms need tests and plan screening that fits your risk.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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On this page
The short answer
Do black, tarry stools mean colon cancer?
Usually not. Stool that is black but firm and ordinary in texture is most often caused by iron tablets or something you ate. Stool that is black, sticky and tar-like, with a strong smell, is different. Doctors call that melaena (often spelt melena), and it usually means blood has been digested on its way down from the stomach or the first part of the small bowel.
Why tarry stool points higher up the gut
Blood turns black when stomach acid and the long journey through the bowel break it down. Blood from the colon, which sits near the end of that journey, usually looks red or maroon instead. So the commonest causes of true melaena are a bleeding stomach ulcer or irritation of the food pipe, not a colon tumour.
Where colon cancer fits in
A tumour in the right side of the colon can bleed slowly and darken the stool, but more often it causes hidden blood loss and anaemia, a low haemoglobin, long before anyone sees a change. That is why tarry stool and unexplained anaemia both need a doctor to find the source.
Tarry black stool you cannot explain needs a doctor today, whatever the cause turns out to be.Most of it is not cancer
What usually turns stool black?
Doctors start with the harmless explanations, then look for bleeding high in the gut. Cancer is on the list, but near the bottom.
Iron tablets and some medicines
Iron tablets, often given in pregnancy or for anaemia, commonly turn stool dark green or black. Bismuth stomach medicines and charcoal tablets do the same.
Medicine-black stool is formed and smells normal. It is not sticky like tar.Dark foods
Jamun, black grapes, blueberries and liquorice can darken stool for a day or so. The colour settles once the food has passed through.
A bleeding ulcer or an irritated stomach
This is the commonest cause of true tarry stool. Painkillers such as diclofenac, ibuprofen and aspirin, a common stomach germ called H. pylori, and heavy drinking can all wear through the stomach lining.
Other causes higher up
- Inflamed food pipe from acid reflux
- Swollen veins in the food pipe from liver disease
- A tear after forceful vomiting
Less often, a growth
A cancer of the stomach, the small bowel or the right side of the colon can bleed slowly and darken the stool. Blood thinners make any of these sources more likely to bleed.
Not sure whether this applies to you?
Ask an oncologist
!
Go today, or see a doctor this week
Go to an emergency department today if your stool is black, sticky and foul-smelling and you are not taking iron, or if black stool comes with dizziness, fainting, a racing heart, breathlessness, pale skin, or vomiting blood or material that looks like coffee grounds. Do not drive yourself. See a doctor this week if dark stool keeps returning, if you have been told your haemoglobin is low, or if you take iron and feel unusually tired.
At the clinic
What will the doctor do about black stools?
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Ask about tablets, food and alcohol
Bring every medicine strip you take, including iron, painkillers, blood thinners and any ayurvedic powders. Many cases are explained in the first few minutes.
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Check your pulse, blood pressure and stool
A fast pulse or low blood pressure means active bleeding. A gentle finger examination of the back passage shows the stool colour directly, which settles whether it is truly tarry.
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Take blood tests
A haemoglobin level shows how much blood has been lost. Iron studies, kidney and liver tests help show where the bleeding came from and how long it has been going on.
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Look inside, starting from the top
An upper endoscopy, a thin camera passed through the mouth into the stomach, is usually the first test for tarry stool. If it is clear, or anaemia remains unexplained, a colonoscopy looks at the colon. At CION, these tests are arranged at accredited partner hospitals.
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Side by side
How is iron-black stool different from tarry stool?
Iron or food
Possible bleeding
Formed, normal texture
Sticky, shiny and tar-like
Usual smell
Unusually strong, foul smell
Dark green or greyish black
Jet black
You feel well
Weak, dizzy, pale or breathless
Started with a tablet or food
Appeared with no explanation
Commonly believed
Four things people say about black stools, and what is true
"It is body heat from spicy food."
Spicy food can upset the stomach, but it does not turn stool black and tarry. If the colour came with no tablet or dark food to explain it, treat it as possible bleeding until a doctor says otherwise.
"It must be piles."
Piles bleed bright red blood onto the paper or into the pan, because the blood is fresh. They do not cause black, tarry stool. Our page on bright red versus dark blood explains why the colour matters.
"I am on iron, so black stool explains itself."
It explains the colour, not the reason you needed iron. In a man, or a woman whose periods have stopped, iron-deficiency anaemia often means slow blood loss from the gut, and the source still needs finding.
"It stopped by itself, so it was nothing."
A bleeding ulcer can stop and start again, sometimes more heavily. A single episode of tarry stool still needs a check, even if you now feel fine.
Being straight with you
What this page cannot tell you
It cannot tell you where your bleeding is coming from, or whether you have cancer. Only an examination, blood tests and usually a camera test can answer that. If you are worried tonight, an emergency department is the right place, not a search engine.
Who this page is not written for
A newborn's first stools are normally sticky and almost black, so this page does not apply to babies. People already under care for liver disease or an ulcer should follow the plan their own doctor gave them. If you see red blood rather than black, start with our page on blood in the stool.
What CION can do
CION does not run an emergency department, so an active bleed belongs at your nearest hospital first. Once you are stable, CION's oncologists can review your reports, arrange endoscopy or colonoscopy through partner hospitals, and explain the results in plain language, in Telugu if you prefer.
Questions we are asked
Common questions about black, tarry stools
What does melena mean?
Melena, or melaena, is the medical word for black, sticky, tar-like stool caused by digested blood. It usually points to bleeding in the food pipe, stomach or first part of the small bowel. It is a sign to be checked the same day, not a diagnosis in itself.
Can iron tablets cause black stools?
Yes, very commonly. Iron turns stool dark green or black, but it stays formed and smells as usual. If your stool becomes sticky or foul-smelling, or you feel faint, do not blame the tablets. Get checked the same day.
Is black stool a sign of colon cancer?
Occasionally, but it is not the typical sign. Most tarry stool comes from bleeding higher up, often an ulcer. Colon cancer more often causes red or dark red blood, a change in bowel habit, or quiet anaemia. Only tests can tell which applies to you.
How long does iron or food darkening last?
Food darkening fades within a day or two of the food passing through. Iron darkening lasts as long as you take the tablets. If the stool stays black after stopping the food or the tablets, ask a doctor.
Can painkillers cause black stools?
They can, indirectly. Painkillers such as diclofenac, ibuprofen and aspirin can cause stomach ulcers that bleed. Older people and those taking them daily for joint pain are most at risk. Do not stop a prescribed blood thinner yourself; ask the doctor who prescribed it.
Which test will the doctor order first?
Usually a blood count and an upper endoscopy, a camera passed through the mouth to see the food pipe and stomach. If that is clear, or anaemia remains unexplained, a colonoscopy usually follows. A stool test alone cannot find the source.
Should I see a gastroenterologist or an oncologist?
For black stool, start with a gastroenterologist or the emergency department, because the cause is usually not cancer. An oncologist becomes involved only if tests find a growth. Our guide to which doctor to see explains the routes.
My elderly parent has black stool and feels weak. What now?
Take them to an emergency department today and carry their medicine list, especially any blood thinners or painkillers. Weakness with black stool can mean significant blood loss. Older people often feel worse than they say, so do not wait to see if it settles.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- MedlinePlus — GI Bleed
- NHS — Bleeding from the bottom (rectal bleeding)
- NHS — Iron deficiency anaemia
- NHS — Symptoms of bowel cancer
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
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Bright red vs dark blood in stoolWhy the colour of blood hints at where it came fromTiredness and iron-deficiency anaemia as the first signThe quiet sign colon cancer more often showsWhen bowel symptoms are an emergencyWhich signs mean go to hospital tonightBlood in stool: what it usually meansIf what you saw was red rather than black
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