Black Stool: Harmless Causes vs. Warning Signs

Black stool can come from iron supplements, bismuth medicines or dark foods such as blueberries, but it can also signal digestive bleeding. Black, sticky, tar-like poop may be melena—digested blood, usually from the upper digestive tract—and needs immediate medical assessment, especially if you feel dizzy, weak or short of breath.

A surprising toilet-bowl color can stop you in your tracks. The useful next step is not to match it against dozens of online photos. It is to check for warning signs, review what you have taken or eaten, and describe the change clearly.

When should you see a doctor for black stool?

Start here before working through possible food or medication causes. You do not need to figure out the diagnosis before asking for help.

Call 911 for signs of serious blood loss

Call 911 if black stool occurs with fainting, confusion, chest pain, severe breathlessness, or cold, clammy skin with a racing pulse. These can be signs of severe bleeding or shock. Do not drive yourself if you feel faint or seriously unwell.

Seek immediate medical care for possible melena

Black, tarry stool needs immediate medical assessment. Vomiting red blood or material that resembles coffee grounds also needs immediate care. An emergency department is appropriate for suspected active bleeding; do not wait for a routine appointment or another bowel movement.

Contact a clinician promptly for unexplained black poop

If your stool is newly black and you cannot confidently connect it to a known food or medicine effect, contact a healthcare professional right away. Tell them if you take blood thinners, including anticoagulants or antiplatelet medicines, because these increase GI bleeding risk.

When calling, lead with the symptom rather than your best guess: “My stool became black today, and I do not know why.” Then describe whether it is sticky, whether you feel unwell, and what medicines you take. Let the clinician help decide where you should be seen.

Which foods and medicines can make poop black?

Not every dark bowel movement contains blood. Some substances change stool color directly, but a plausible explanation should not override warning signs.

Iron supplements

Iron tablets commonly make stool darker or black. They can also cause constipation, diarrhea, nausea or stomach discomfort. A change after starting iron can therefore be an expected side effect, rather than evidence that the supplement is damaging your gut.

The distinction matters: black stool alone is not the same as black, tarry stool. MedlinePlus advises contacting your provider right away if stool is tarry as well as black, has red streaks, or occurs with stomach cramps or sharp pain.

Do not stop prescribed iron solely because of an expected color change without discussing it with your clinician. If side effects are difficult to manage, ask whether a different preparation or treatment plan would suit you.

For that conversation, write down the product name, when you started it and when the dark stool appeared. “A supplement” is less helpful than the actual label, so bring the bottle or a photo of it.

Bismuth medicines, including Pepto-Bismol

Bismuth subsalicylate can turn stool black and may also darken your tongue. The NHS describes these color changes as harmless medication effects that usually go away on their own. If they persist or bother you, speak with a pharmacist or doctor.

Check the active ingredient on your stomach remedy rather than relying only on its brand name. Include over-the-counter products when someone asks what medicines you take; they count too.

Do not take extra bismuth to treat unexplained black stool. Pepto-Bismol may be unsuitable for people with blood in their stool, stomach ulcers or clotting disorders, and it can interact with blood thinners. Ask a pharmacist or clinician if you are unsure.

Blueberries and other dark foods

Blueberries, black licorice and blood sausage can make stool appear black. Activated charcoal can do the same. A recent food history is useful, but it does not prove that a dark bowel movement is harmless.

Try making a quick list rather than a perfect food diary: what you ate, roughly how much, and when. Include smoothies, snacks and supplements—not just meals. The goal is to give your clinician useful context, not to put every blueberry on trial.

For a broader look at color changes, see our poop color chart.

What does melena look like compared with dark stool?

Melena typically looks jet black, sticky and tar-like, often with a notably strong, unpleasant smell. It happens when blood changes as it travels through the digestive tract. The source is usually the esophagus, stomach or upper small intestine.

Use the comparison below to describe what you notice—not to clear yourself of bleeding at home.

Possible explanation	Helpful clues	Safer next step
Iron-related darkening	Color changed during iron treatment	Discuss uncertainty or troublesome side effects with your prescriber
Bismuth-related darkening	Recent bismuth use, sometimes with a dark tongue	Ask a pharmacist if the change persists
Food-related darkening	Recent blueberries, black licorice or another dark food	Note the timing, but do not dismiss warning signs
Possible melena	Black, sticky, tar-like stool, sometimes with a strong odor	Seek immediate medical assessment

These are clues, not a diagnostic checklist. Iron and bismuth can explain color changes, while tarry stool or feeling unwell still warrants medical attention.

Melena does not always follow the textbook description. It can be loose or watery, and a smaller amount of bleeding may look dark brown rather than jet black. Appearance and smell alone cannot reliably exclude blood.

If you cannot decide whether “tarry” fits, say exactly that when you call. You are not expected to know medical vocabulary or make a diagnosis from the toilet bowl.

What causes upper GI bleeding?

A bleeding peptic ulcer is one possible cause. Ulcers are sores in the stomach or the first part of the small intestine. Common contributors include infection with H. pylori bacteria and nonsteroidal anti-inflammatory drugs, or NSAIDs, such as ibuprofen and naproxen.

Other causes include inflammation of the stomach or esophagus, a tear after severe vomiting, and enlarged veins called varices, often associated with cirrhosis. These are different conditions that need different treatments; black stool alone cannot tell you which is present.

Bleeding can also be slow or come and go. Over time, ongoing blood loss may cause anemia, with symptoms such as unusual tiredness, pale skin or shortness of breath. Tell your clinician about these changes even if they developed gradually.

You do not need to memorize this list. The practical point is that possible bleeding calls for an assessment of its source, not just a plan to make the stool look brown again.