Black Stool: Iron, Bismuth or Bleeding?

If your poo has turned black, think back over what you’ve recently eaten or taken. Iron tablets, bismuth stomach medicine and a few dark foods can all do it, but so can blood from high up in the gut, and looking alone can’t rule that out.

Medicines and foods that darken stool

Cause	What the source says
Iron tablets	The NHS says darker than usual poo is common with iron and usually nothing to worry about
Bismuth medicines such as Pepto-Bismol	MedlinePlus says the darkening of stool and tongue is harmless and usually goes away a few days after you stop
Black licorice, blueberries, blood sausage	MedlinePlus lists all three as foods that can cause black stools
Activated charcoal	Also on the MedlinePlus list

Even when one of these fits, the NHS says very dark or black poo is sometimes a sign of something else, and a GP can do a test if you’re worried. MedlinePlus agrees that a provider can test the stool with a chemical to check for blood. Its bismuth subsalicylate page says to ask your doctor before taking the medicine if you have ever had an ulcer, a bleeding problem, or bloody or blackened stools.

The NHS ferrous sulfate page says to talk to a doctor or call 111 straight away if your poo is black, tar-like or red, has blood in or on it, or you feel unwell in any other way. That advice sits on the iron medicine’s own page, so it applies while you’re taking iron.

When black means bleeding

MedlinePlus says black or tarry stool with a foul smell is a sign of a problem in the upper digestive tract, most often bleeding in the esophagus, stomach or first part of the small intestine. The blood is darker because it gets digested on its way through, and the medical term is melena. Red or fresh blood most often comes from lower down, in the colon, rectum or anus; the blood in stool guide covers that.

Peptic ulcers are the most common cause of acute upper GI bleeding, according to MedlinePlus. NIDDK adds gastritis, esophagitis, varices (enlarged veins in the esophagus or stomach, most often related to cirrhosis), Mallory-Weiss tears (often caused by severe vomiting), and benign tumors or cancer. It names H. pylori infection, NSAIDs such as aspirin, ibuprofen and naproxen, and blood thinners as the most common causes of peptic ulcers. MedlinePlus says anticoagulant and antiplatelet medicines increase the risk of GI bleeding.

NIDDK says slow, chronic bleeding can make stool sometimes black and tarry, sometimes mixed with bright red blood, and over time can cause anemia, with tiredness, pale skin and shortness of breath.

When to get help

MedlinePlus says to contact your provider right away if you notice blood or changes in the color of your stool, vomit blood, or feel dizzy or lightheaded. The NHS rectal bleeding page says to ask for an urgent GP appointment or contact NHS 111 if your poo is black or dark red, or you have bloody diarrhoea.

The NHS stomach ulcer and rectal bleeding pages say to call 999 or go to A&E if:

• your poo has blood in it, or is black, sticky and very smelly
• you vomit bright red blood, or your vomit looks like ground coffee
• you have severe tummy pain
• you’re bleeding non-stop, or there’s a lot of blood, such as red toilet water or large clots

NIDDK says severe acute GI bleeding can cause shock, which is life-threatening and needs emergency help right away. Signs include confusion or unconsciousness, a fast heart rate, pale skin, cold hands and feet, and sweating. The NHS says not to drive yourself to A&E. Outside the UK, routes differ: MedlinePlus, a US government health site, lists black, tarry stools among reasons to call 911 or go to the nearest emergency room. Use your local emergency number for black, tarry stool and for the emergency signs. This article is general information, not medical advice.

A made-up log

Day	Stool	Notes
Mon	Type 4, darker than usual	Started iron tablets Sunday
Tue	Type 4, darker than usual	Feels fine
Wed	Type 5, darker than usual	Feels fine
Thu	Black, tar-like, sticky, very smelly	Lightheaded on standing

Monday to Wednesday match what the NHS says about iron: darker poo is common and usually nothing to worry about. Thursday is different. Black or tar-like poo is on the NHS ferrous sulfate page’s straight-away list, black, sticky, very smelly poo is on the NHS stomach ulcer page’s 999 list, and MedlinePlus says to contact a provider right away if you feel dizzy or lightheaded.

MedlinePlus lists what a provider may ask: blood thinners or NSAIDs you take, whether you’ve had black licorice, iron, Pepto-Bismol or blueberries, how many episodes there were, when it started, weight loss, and other symptoms such as abdominal pain, vomiting blood or fever.

How bellySignal helps

bellySignal logs stool color next to the Bristol type (1 to 7), with a flag for blood or mucus. Food triggers and notes let you put the iron tablet, the Pepto-Bismol dose or the blueberries beside the stool they might explain. The app can’t tell digested blood from iron. Other colors are in the stool color guide.

Your diary is stored on your device, and you can start logging without signing up. Optional login, health platform sync, and export are available. Some health values, including stool type, symptoms, and manually entered weight, are also sent to PostHog, which is enabled by default. The analytics switch in settings controls PostHog only. Firebase Analytics receives usage events without those health values and remains active. Privacy policy.