Why Are Your Stools Black or Dark and When Should You Be Concerned
Last updated: 19 Aug 2026
You've been noticing your stools look darker than normal. Not bright red blood, but noticeably darker, almost black in some cases. You're not sure if you should worry or not. Should you rush to emergency? Should you call your GP? Or is it probably just something you ate, something that'll pass on its own? The uncertainty is frustrating because you want to know whether this is serious or not.
The honest answer is that dark stools can mean several different things, and most of them are either benign or highly treatable. But they do warrant investigation. Not panic, but prompt attention.
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Understanding What Dark Stools Actually Mean
Your stool colour tells a story about what's happening in your digestive system. Normal stools are brown, which comes from bilirubin, a byproduct of red blood cell breakdown. When something changes that story, the colour changes too.
Dark or black stools specifically signal one of two things: either something has altered your stool colour without involving blood, or there is blood in your stool that's coming from somewhere in your upper digestive tract.
The distinction matters clinically because it determines whether you're dealing with something benign (which can usually wait a few days for investigation) or something that warrants more urgent attention (which needs investigation within days).
Benign Causes of Dark Stools
Not all dark stools involve bleeding, and that's important to understand before you spiral into worry.
Iron Supplements
Iron supplements are probably the most common benign cause of very dark, almost black stools. If you've recently started taking iron tablets, iron-fortified vitamins, or certain prenatal vitamins with iron, this is almost certainly the explanation. The iron passes through your digestive system largely unabsorbed and colours your stool very dark.
This is completely harmless. It's not blood. It's not an indication of anything wrong. It's just what happens when you take iron supplements. If the dark stools coincide with starting iron, you can be relatively confident this is the cause.
Bismuth Subsalicylate (Pepto-Bismol) and Other Medications
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, commonly causes dark grey or black stools. Other medications can too. Some antacids, antibiotics, and even certain foods can darken stool without involving any bleeding.
Again, this is benign. It's just a side effect of the medication or food passing through your system and affecting colour.
Dietary Causes
Certain foods naturally cause dark stools. Beets, blueberries, dark chocolate, activated charcoal, dark licorice. If you've been eating any of these in quantity, that's often the explanation. The dark pigments from these foods pass through your system and colour your stool.
Other Benign Causes
Dehydration can make stools darker because there's less water to dilute the bilirubin. Certain probiotics or supplements can affect colour. Sometimes it's simply that your stools are naturally on the darker side of the brown spectrum and you're noticing it for the first time.
Concerning Causes That Warrant Investigation
While many dark stools are benign, some indicate bleeding in the digestive tract. The location and nature of the bleeding determines the urgency and the investigation pathway.
Upper GI Bleeding (Melena)
Bleeding from the stomach, oesophagus, or upper small intestine typically produces a distinctive appearance called melena. This stool is very dark, tarry, shiny, and often has a putrid smell. It looks noticeably different from ordinary dark stool.
Melena occurs because the blood travels through the digestive tract and gets partially digested, which darkens it. If your dark stools have this distinctive tarry appearance, this is a sign of upper GI bleeding and warrants prompt investigation.
Common causes of upper GI bleeding include peptic ulcers (often caused by H. pylori), gastritis, acid reflux, or less commonly, varices or other vascular issues.
The important thing to understand is that upper GI bleeding doesn't always cause dramatic symptoms. You might not have severe abdominal pain. You might not feel particularly unwell. The dark stools might be the primary sign. This is exactly why investigating dark stools matters, even if you don't have other obvious symptoms.
Lower GI Bleeding
Bleeding from the colon or lower intestines typically appears as maroon-coloured stools or bright red blood, not black. So if you're seeing very dark stools, lower GI bleeding is less likely. But it's worth mentioning for context.
Bleeding from Ulcers, Polyps, or Inflammation
Peptic ulcers (often H. pylori-related) can bleed slowly, producing dark stools without severe pain. Polyps can bleed. Inflammatory conditions like Crohn's disease can cause bleeding. In all these cases, the bleeding might be relatively asymptomatic except for the stool colour change.
Early detection matters because many of these causes are either preventable or highly treatable when caught.
How to Tell the Difference
Here's a practical way to assess what you're dealing with.
- Timing - Did your dark stools start right after you began iron supplements, Pepto-Bismol, or after eating certain foods? If yes, benign cause is likely.
- Appearance - Is the stool distinctly tarry and shiny black, unlike anything you've seen before? Or is it just darker brown than usual? Tarry appearance is more suggestive of concerning bleeding. Regular dark brown is more likely benign.
- Symptoms - Do you have abdominal pain, nausea, vomiting, or signs of anaemia (fatigue, dizziness, shortness of breath)? Multiple symptoms alongside dark stools suggest investigation is more urgent.
- Consistency - Has your stool become looser alongside the colour change? Loose, dark stools can suggest bleeding. Firm but dark stools are more likely benign.
None of these distinctions are diagnostic on their own. But together they help you assess whether you're dealing with something that probably needs investigation within days or something that might warrant more urgent attention.
When to See a Doctor and Get Tested
Book a consultation promptly (within days) if you experience any of the following:
- Noticeably dark or black stools that represent a change from your normal pattern
- Tarry, shiny black stools that look distinctly different from ordinary dark stool
- Dark stools alongside abdominal pain, nausea, or vomiting
- Dark stools alongside signs of anaemia (unusual fatigue, dizziness, shortness of breath, pale skin)
- Dark stools that persist for more than a few days despite no obvious benign cause
- Dark stools that return repeatedly over weeks
- Dark stools after an injury or event that might have caused internal bleeding
- Seek urgent attention (same day or emergency) if you have:
- Tarry black stools with severe abdominal pain
- Dark stools alongside vomiting blood
- Signs of severe anaemia (severe dizziness, fainting, chest pain, extreme shortness of breath)
- Tarry stools alongside signs of shock (rapid heart rate, severe weakness, confusion)
Most cases of dark stools fall into the "book a routine appointment within days" category rather than the emergency category. But some warrant more urgent evaluation. Trust your instinct. If something feels off, get checked promptly rather than waiting.
What Tests Can Help
If you come to our clinic with dark stools, here's what our Doctors might recommend depending on your symptom pattern and history.
- Faecal Occult Blood Test is typically the first test. It detects hidden blood in your stool that isn't visible to the naked eye. This tells our Doctors whether bleeding is actually occurring or whether the dark colour has a benign explanation. Results are usually available within 1 to 3 working days.
- Full Blood Count helps identify whether you have anaemia, which would suggest blood loss has been occurring. If you're anaemic alongside dark stools, that's another clue that bleeding is happening.
- Iron Level and Ferritin Test provides additional information about whether chronic blood loss is affecting your iron stores. This is particularly relevant if the anaemia is already established.
- H. Pylori Testing becomes relevant if the faecal occult blood test is positive and our Doctors suspect H. pylori-related bleeding (peptic ulcers or gastritis). A urea breath test takes about 30 minutes with results available within 3-5 working days. Alternatively, an H. pylori stool antigen test detects the infection through a stool sample. Identifying H. pylori as the cause changes the treatment approach entirely, antibiotics can clear the infection and stop the bleeding.
- Liver Function Test might be recommended if our Doctors are concerned about bleeding from varices or other liver-related bleeding sources, though this is less common.
For patients with multiple digestive concerns alongside dark stools, a comprehensive gut health screening can cover the key investigations in a single visit, including H. pylori testing, stool analysis, and blood work.
If the faecal occult blood test comes back positive or if other signs point to upper GI bleeding, our Doctors will arrange a referral to a gastroenterologist for a gastroscopy (endoscopy). This allows direct visualisation of the oesophagus, stomach, and upper small intestine to identify the bleeding source.
What Happens After Testing
If the faecal occult blood test comes back negative and there's no sign of anaemia or other concerning features, the dark stools are almost certainly from a benign cause. You can have confidence that bleeding isn't occurring. If you're taking iron supplements or Pepto-Bismol, you have your answer. If not, dietary or other benign causes are likely.
If the test comes back positive or if signs point to bleeding, further investigation with gastroscopy is warranted. This procedure allows our specialist colleague to see exactly where the bleeding is coming from and often to treat it directly.
For H. pylori-related ulcers or gastritis causing bleeding, treatment with antibiotics and acid-suppressing medication typically stops the bleeding and allows the ulcer to heal. Most patients see improvement within days of starting treatment.
For bleeding polyps, these can usually be removed during colonoscopy. For other causes of bleeding, management depends on the specific diagnosis but is generally straightforward once identified.
Test and Manage Your Gut Health
Dark stools are a signal worth investigating promptly, but not necessarily an emergency to panic about. A simple fecal occult blood test can determine whether blood is actually present, giving you either reassurance or clarity on next steps.
Your body is communicating through changes in stool colour. The question is whether you listen and investigate, or ignore it and spend weeks wondering. Most of the time the answer is reassuring. Sometimes it's important. Either way, finding out is simpler than living in uncertainty.
Test and Manage Your Gut Health
Patient-first, Holistic, Dedicated Healthcare
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