Black, tarry stool has a medical name: melaena. It usually means blood from higher up your gut, digested on its way out, and it needs medical attention. Most black stool is nothing like that: iron tablets, Pepto-Bismol, activated charcoal and dark foods like black liquorice or blueberries can all turn your colour and consistency black too, and it clears once the cause does [1][5].
See a doctor the same day if your stool is black and tarry with no obvious cause. Go straight to A&E or call 999 if it comes with vomiting blood, fainting, dizziness or weakness, a racing heartbeat, or severe abdominal pain [5][2].
What Causes Black Stool?
Most black stool is simply something you swallowed. Iron, bismuth, charcoal or a very dark food passes through and darkens colour without changing texture or smell [1]. If you're building a gut health baseline, this is worth logging alongside what you ate or took and when, similar to any of the other gut health metrics worth tracking.
The rarer cause is blood from your oesophagus, stomach or upper small intestine, partially digested on its way out. This is melaena, and it's a recognised sign of possible gastrointestinal bleeding that needs prompt medical evaluation [2][5]. Regular NSAID or aspirin use is a common, often unrecognised, contributor to this kind of bleeding [3], which is one of several risk factors worth mentioning to a doctor alongside the wider signs your gut health is off.
What Is Melaena, and How Is It Different?
Melaena is black, sticky, tar like stool caused by digested blood, distinct from stool simply darkened by iron or food, and it's a different question from the more common causes of poor gut health. The difference is texture and smell, not colour, the same distinction that matters when reading mucus in stool, and one reason stool changes like this shouldn't be judged by shade alone.
Harmless Colour Change vs Melaena: How to Tell the Difference
Here's the fastest way to tell them apart.
| Something you ate or took | Possible bleeding (melaena) | |
|---|---|---|
| Texture and smell | Normal for you | Sticky, tarry, glossy, and distinctly, strongly offensive |
| Timing | Starts within a day or two of the supplement, medicine or food, and stops when you stop | Appears without an obvious cause |
| What to do | Usually nothing | See a doctor the same day. Call 999 or go to A&E for vomiting blood, fainting, dizziness, a racing heartbeat, or severe pain |
When Black Stool Needs Urgent Attention
Anticoagulants, stomach or duodenal ulcers, gastritis, oesophageal varices and a Mallory Weiss tear from forceful vomiting can all contribute to melaena alongside NSAID use [6]. Call 999 or go to A&E straight away if black stool comes with vomiting blood or vomit that looks like coffee grounds, fainting, dizziness or weakness, a rapid or irregular heartbeat, severe abdominal pain, or breathlessness [5][6]. See a doctor the same day, even without those symptoms, if you're on anticoagulants, take NSAIDs regularly, have had unexplained weight loss, or the black stool keeps recurring. Small black specks in otherwise normal stool are usually just food, poppy seeds or blackberry pips, which is a different and much less urgent question. The full stool colour chart and guide to pale stool cover the other changes worth knowing.
How long does black stool from iron tablets last?
Usually as long as you're taking them, clearing within days of stopping [4]. If it persists well after, get it checked.
Is dark brown stool the same as black?
No. Stool colour varies daily and dark brown is normal, as the Bristol Stool Chart shows. Melaena is unmistakably black and tarry, not simply dark.
Black stool is usually harmless: iron, Pepto-Bismol, charcoal or dark food, clearing once the cause does. What separates that from melaena is texture and smell, not colour, so don't try to rule bleeding out by looking at it. See a doctor the same day for unexplained black, tarry stool, and go straight to A&E or call 999 if it comes with vomiting blood, fainting, dizziness, a racing heartbeat, or severe pain.
References
- NHS. Poo: what's normal and what's not. NHS.uk.
- NHS. Bleeding from the bottom (rectal bleeding). NHS.uk.
- Lanas, A., & Chan, F. K. L. (2017). Peptic ulcer disease. The Lancet, 390(10094), 613-624.
- NHS. Side effects of ferrous sulfate. NHS.uk.
- NHS. Pepto-Bismol (bismuth subsalicylate). NHS.uk.
- National Library of Medicine (MedlinePlus). Black or tarry stools. medlineplus.gov.
This content is for informational purposes only and does not constitute medical advice. Gutsi is a wellness tracking device and is not intended to diagnose, treat, cure, or prevent any condition. If you have concerns about your digestive health, please speak to a qualified healthcare professional.
