Medically reviewed guide · Last updated: 24 October 2026 · Reading time: approx. 11 min

Black Stool: Iron, Bleeding or Food?

The most common causes compared

The causes in detail

This article helps you with one question: is the stool black because something has coloured it — or because there is bleeding? Red blood on the toilet paper, dark red traces in the stool and bowel cancer screening are covered in detail in the article on blood in the stool. If, on the other hand, the stool is strikingly pale or clay-coloured, the article on pale stool will help.

Iron preparations: the most common harmless explanation

If you take iron preparations, part of the iron leaves the body again — as dark iron compounds that turn the stool black, dark grey or greenish-black. The summaries of product characteristics list this as a common, harmless side effect, and it usually begins within a few days of starting. Typically, the stool is formed or normally soft, not sticky and shiny, and does not smell any different from usual. Constipation or a queasy feeling in the stomach often comes with it.

There is one important catch: iron is often prescribed for iron deficiency — and iron deficiency without a clear cause can itself come from slow bleeding in the gastrointestinal tract. The black colour from the iron can mask such bleeding. That is why, with unexplained iron deficiency anaemia, looking for the cause is part of the work-up, especially in men and in women after the menopause.

Bismuth and medicinal charcoal

Bismuth is part of a quadruple therapy against the stomach bacterium Helicobacter pylori and is found in some stomach remedies, especially those common abroad. It turns the stool greyish-black to black, and sometimes the tongue as well. Medicinal charcoal, which is used for diarrhoea or poisoning, also makes the stool jet-black. Both are harmless and disappear a few days after you stop taking them.

Food: blueberries, black pudding and the like

Larger amounts of blueberries or blackberries, black pudding, black liquorice, pasta or risotto with squid ink, and beetroot can turn the stool very dark — from bluish-black to dark reddish-brown. Spinach and other green leafy vegetables tend to make it dark green. Beetroot often turns the urine reddish too. The discolouration only lasts as long as the food takes to pass through the gut, which is usually one to two days. If you have eaten none of these, or the colour lasts longer, you should look further.

Swallowed blood

Blood that you swallow is broken down in the gut in exactly the same way as bleeding in the stomach. After a heavy nosebleed, after a tooth has been pulled, after tonsil surgery or with heavily bleeding gums, the stool can therefore be black one to two days later. Breastfed babies sometimes swallow blood from their mother's sore nipples. If the source is clear and the bleeding stops, the stool usually returns to normal on its own — but repeated nosebleeds while taking blood thinners should be looked at by a doctor.

Tarry stool: bleeding in the upper digestive tract

Genuine tarry stool (melaena) develops when blood from the oesophagus, stomach or duodenum is broken down by stomach acid and gut bacteria. It is shiny black, sticky like tar, often mushy, and has a strikingly pungent, putrid smell. Tarry stool means that a significant amount of blood has already reached the gut — and it can continue to colour the stool for a few days, even after the bleeding has stopped. Less often, dark to black stool comes from the small intestine or the right side of the large intestine. Conversely, very heavy bleeding from the stomach can pass through the gut so quickly that red blood appears.

The most common source is a stomach or duodenal ulcer. The two most important triggers are a Helicobacter infection and anti-inflammatory painkillers, including aspirin. Ulcers caused by painkillers often bleed without hurting beforehand. Other possible causes include erosive gastritis, inflammation of the oesophagus due to reflux, a tear in the lining after violent vomiting (Mallory-Weiss syndrome) and tumours such as stomach cancer, which more often bleed slowly and unnoticed.

Varicose veins of the oesophagus in liver cirrhosis

In advanced liver cirrhosis, blood backs up in front of the liver, and widened veins form in the oesophagus (oesophageal varices). If they rupture, the bleeding is usually heavy — with vomiting of blood, tarry stool and rapidly developing circulatory weakness. This is a life-threatening emergency. If you have known liver disease, you should therefore never wait and see with black stool.

Newborns: meconium is normal

In the first one to three days of life, newborns pass meconium: thick, sticky and greenish-black. This is normal. After that, the stool turns greenish-brown via transitional stools and finally yellow. Unexplained black stool in an older baby or toddler belongs in the paediatric practice.

Self-tests: first clues at home

These observations do not replace a diagnosis — they help you judge how urgently things need to be checked. If you feel weak or dizzy, or you see blood in your vomit: do not keep watching, get help.

• The consistency check: formed, matt black or greenish-black stool fits better with iron, bismuth or food. Shiny, sticky, mushy stool that is hard to flush away fits better with tarry stool.
• The smell check: tarry stool smells strikingly pungent and different from your usual stool. Stool coloured by iron or food usually smells the same as always.
• The 48-hour look-back: what have you eaten and taken in the last two days? Blueberries, black pudding, liquorice, charcoal tablets, a new iron preparation or Helicobacter treatment often explain the colour straight away.
• The circulation check: stand up slowly and notice whether you feel dizzy or your vision goes dark, whether your heart is racing or whether you are unusually pale and in a cold sweat. These are signs of blood loss — in that case, stop testing.
• The bleeding source check: have you had a nosebleed, dental treatment or bleeding gums? Then swallowed blood may be the explanation.
• The medicines check: are you taking ibuprofen, diclofenac, aspirin, a blood thinner or cortisone — and perhaps several of these at the same time? Then a stomach bleed is more likely, and the threshold for seeing a doctor is lower.

Warning signs: when not to wait

• Shiny black, sticky, foul-smelling stool not explained by iron, bismuth, charcoal or food
• Dizziness, fainting, a racing heart, paleness, cold sweat or unusual breathlessness on exertion
• Vomiting blood or dark brown stomach contents that look like coffee grounds
• Black stool while taking blood thinners such as apixaban, rivaroxaban or phenprocoumon, or two antiplatelet medicines
• Known liver cirrhosis, a previous stomach bleed or a known stomach ulcer
• Severe abdominal pain, a hard, tense abdomen
• Black stool together with tiredness, reduced stamina or unintended weight loss over weeks

The treatment pathway: step by step

What happens next depends on whether the colour has a harmless explanation or whether bleeding is a possibility.

1. Check for a harmless explanation. If the black stool clearly fits an iron preparation, bismuth, charcoal or food and you feel well, it is usually enough to keep an eye on the colour and mention it at your next appointment. After food, it should disappear within two to three days.
2. Initial medical examination. If tarry stool is suspected, pulse and blood pressure are measured, the abdomen is palpated and the rectum is often examined with a finger to see the stool colour directly. A blood count shows whether the red blood pigment (haemoglobin) has fallen; clotting and kidney values complete the picture.
3. Assess the risk. Using laboratory values, pulse, blood pressure and previous illnesses, the team assesses how urgently action is needed. Guidelines use standardised risk scores for this.
4. Gastroscopy. If bleeding from the upper digestive tract is suspected, a gastroscopy follows — according to the guideline usually within 24 hours, and sooner if the circulation is unstable. A bleeding source can often be stopped directly during the procedure, for example with clips or injections. If nothing is found at the top, a colonoscopy or an examination of the small intestine may follow.
5. Treat the cause. An ulcer is treated with acid blockers, and a Helicobacter infection is eliminated with combination therapy. Whether painkillers, aspirin or blood thinners are paused, replaced or continued with stomach protection is always decided by the practice treating you — depending on why you need them.

The medication angle: colouring or causing bleeding

With black stool, medicines play a double role: some colour it harmlessly, others increase the risk of bleeding in the stomach. An up-to-date medication plan helps you and the practice tell the two apart.

Medicines that colour harmlessly

Iron preparations, bismuth-containing combinations against Helicobacter and medicinal charcoal turn the stool black without damaging the lining of the stomach. If you start one of them, note down the start date — then the colour will not come as a surprise the next time you go to the toilet. Still, stay alert: if your stool suddenly becomes sticky, shiny and foul-smelling while taking iron, or you feel weak, the iron no longer explains it.

Anti-inflammatory painkillers and aspirin

Active substances such as ibuprofen, diclofenac, naproxen and aspirin inhibit protective messenger substances in the stomach lining. Ulcers and bleeding are therefore a known, though not common, side effect — the risk rises with dose, duration and age, with previous ulcers and in combination with cortisone, blood thinners or certain antidepressants (SSRIs). According to the package leaflet, you should generally not take over-the-counter painkillers for longer than three to four days in a row without medical advice. When stomach protection such as pantoprazole makes sense is explained in the guide Stomach problems caused by medicines.

Blood thinners and antiplatelet medicines

Anticoagulants such as apixaban, rivaroxaban, edoxaban, dabigatran or phenprocoumon do not usually cause ulcers, but they make an existing weak spot bleed more heavily. The same applies to antiplatelet medicines such as clopidogrel or ticagrelor, especially in dual combination with aspirin after a stent. Black stool while taking these medicines is always a reason for prompt assessment.

How to prevent a stomach bleed

You do not need to prevent harmlessly black stool. But much of what lies behind genuine tarry stool can be prevented.

• Use painkillers in a targeted way — for as short a time and at as low a dose as possible, and never two anti-inflammatory medicines at the same time. The painkiller comparison shows what alternatives there are.
• Discuss stomach protection — if you take aspirin, painkillers or blood thinners long term and are older or have already had an ulcer, you should clarify with the practice whether an acid blocker makes sense.
• Have Helicobacter treated — before planned long-term treatment with aspirin or painkillers and after an ulcer, a test for the stomach bacterium can be worthwhile.
• Alcohol in moderation — alcohol irritates the stomach lining, increases the effect of painkillers on the stomach and is the most common cause of liver cirrhosis.
• Note down new products — record iron, bismuth or charcoal with the start date in your medication plan. That way you can place a discolouration immediately and notice when something does not fit.

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