Bloody stool has a wide range of causes, from something as minor as a small hemorrhoid to something as serious as colorectal cancer. The color and appearance of the blood offer important clues: bright red blood typically comes from the lower digestive tract (colon, rectum, or anus), while dark, tarry, almost black stool usually signals bleeding higher up, in the stomach or upper intestine. About 90% of significant GI bleeding episodes originate above the colon.
What the Color of Blood Tells You
Bright red blood on toilet paper, in the bowl, or mixed into stool points toward a source in the colon or rectum. Doctors call this hematochezia. It’s the most common type people notice, and in many cases the cause is something treatable near the end of the digestive tract.
Dark, sticky, tar-like stool, sometimes called melena, looks and smells distinctly different. Blood that travels from the stomach or upper small intestine gets partially digested along the way, turning it black. If your stool looks like this, the bleeding source is almost always above the colon. One exception: very rapid, heavy bleeding from an upper source (like a peptic ulcer hitting an artery) can move through the gut so fast that it still appears red when it comes out.
Hemorrhoids and Anal Fissures
These two causes account for a large share of the bright red blood people see. They’re different problems, though, and they feel different too.
Hemorrhoids are swollen blood vessels in or around the anus. Internal hemorrhoids often bleed without causing pain at all. You might notice bright red streaks on toilet paper or drops in the bowl after a bowel movement and feel nothing unusual. External hemorrhoids can itch or cause mild discomfort, but the hallmark of internal hemorrhoids is painless bleeding.
Anal fissures are small tears in the skin lining the anus, usually caused by passing hard or large stools. Unlike hemorrhoids, fissures hurt. The pain is typically sharp during a bowel movement and can linger afterward. You’ll see blood when you wipe or on the surface of the stool. Most fissures heal on their own within a few weeks with increased fiber and water intake.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease cause chronic inflammation in the digestive tract, and both can produce bloody stool. Ulcerative colitis almost always involves the rectum and colon, so bloody diarrhea is one of its defining symptoms. Crohn’s disease can affect any part of the GI tract, so bleeding is less predictable but still occurs. Clinically significant lower GI bleeding happens in roughly 1 to 6% of patients with either condition, but milder blood in the stool is far more common during flares.
Other signs that point toward IBD include persistent diarrhea lasting weeks, abdominal cramping, urgency, fatigue, and unintended weight loss. These conditions are chronic and require ongoing management, but many people achieve long stretches of remission with treatment.
Bacterial Infections
Several types of food poisoning can cause bloody diarrhea, often alongside fever, cramping, and nausea. The most common culprits:
- Shiga toxin-producing E. coli (STEC): Diarrhea starts about three days after exposure, initially watery, then turns bloody in about 90% of cases within a few more days.
- Shigella: After a one-to-four-day incubation period, symptoms can progress from watery diarrhea to frequent small-volume stools with visible blood and mucus.
- Campylobacter: One of the most common causes of bacterial gastroenteritis worldwide. It often produces bloody diarrhea along with fever and abdominal pain.
- Yersinia: Less common but also capable of causing bloody diarrhea, usually from contaminated pork or untreated water.
Notably, Salmonella, despite being one of the best-known food poisoning bacteria, typically causes non-bloody diarrhea. Most bacterial GI infections resolve on their own within a week, though STEC infections require careful monitoring because they can, in rare cases, damage the kidneys.
NSAIDs and Other Medications
Over-the-counter painkillers like ibuprofen, naproxen, and aspirin are among the most common medication-related causes of GI bleeding. These drugs work by blocking an enzyme involved in inflammation, but that same enzyme also helps maintain the protective mucus lining of the stomach and small intestine. Without that barrier, stomach acid can erode the tissue and cause ulcers that bleed.
The risk increases with higher doses and longer use. Stomach ulcers caused by these medications are more common than duodenal ones, and they can bleed silently for a while before you notice dark or tarry stools. Blood thinners (anticoagulants) don’t cause bleeding on their own, but they make any existing source of bleeding harder for the body to stop, so even a minor issue like a small hemorrhoid can produce more noticeable blood.
Colorectal Polyps and Cancer
Polyps are small growths on the inner lining of the colon. Most are harmless, but some can bleed, and over years, certain types can develop into colorectal cancer. Cancer itself can cause bleeding that’s either visible or too small to see without testing (occult bleeding). Other warning signs include a persistent change in bowel habits, unexplained weight loss, or a feeling that the bowel doesn’t fully empty.
The U.S. Preventive Services Task Force recommends that average-risk adults begin colorectal cancer screening at age 45. Screening continues through age 75 and can be done through stool-based tests or colonoscopy, depending on the approach you and your doctor choose. People with a family history of colorectal cancer or a personal history of inflammatory bowel disease may need to start earlier.
Foods That Mimic Bloody Stool
Before assuming the worst, consider what you’ve eaten in the past day or two. Beets are the most well-known culprit: they can turn both stool and urine a startling shade of red. Red gelatin, tomato soup, red-skinned fruits, and red food dye can do the same. Iron supplements and bismuth subsalicylate (the active ingredient in some stomach-relief medications) turn stool very dark or black, mimicking the appearance of upper GI bleeding. These are harmless, but if you’re unsure whether what you’re seeing is food-related, a simple stool test can check for the presence of actual blood.
How Doctors Find the Source
Figuring out where bleeding is coming from usually starts with basic tests. A stool sample can confirm whether blood is present, especially when it’s not visible to the naked eye. Blood tests help determine how much blood you’ve lost and whether you’ve become anemic.
The most informative tool is endoscopy: a flexible tube with a camera that lets a doctor see the lining of the digestive tract directly. An upper endoscopy examines the esophagus, stomach, and the beginning of the small intestine. A colonoscopy examines the rectum and entire colon. In some cases, doctors can treat the source of bleeding during the same procedure, using heat, clips, or other techniques to stop it.
When standard endoscopy doesn’t find the source, other options include capsule endoscopy (swallowing a pill-sized camera that photographs the small intestine as it passes through), CT scans with contrast dye to visualize blood vessels, or radionuclide scanning, which uses a small amount of radioactive tracer to pinpoint active bleeding on imaging.
Signs That Need Emergency Attention
A small amount of bright red blood on toilet paper after straining is common and rarely an emergency. But heavy or continuous rectal bleeding, especially with severe abdominal pain, warrants an immediate trip to the emergency room.
Certain symptoms alongside bloody stool suggest significant blood loss and require a 911 call: dizziness or lightheadedness when standing, rapid shallow breathing, fainting, confusion, blurred vision, cold or clammy skin, or very low urine output. These are signs the body is struggling to compensate for the volume of blood lost, and they can develop quickly with arterial bleeding from an ulcer or other upper GI source.