Blood in your stool usually comes from somewhere along your digestive tract, and the color of the blood is the single biggest clue to where. Bright red blood typically originates in the colon or rectum. Dark, tarry, sticky stool points to bleeding higher up, in the stomach or small intestine, where blood has been partially digested on its way through. Most causes are not life-threatening, but some are, so understanding what you’re seeing matters.
What the Color Tells You
Bright red blood on toilet paper, in the bowl, or coating the stool means the bleeding source is close to the exit, usually the rectum or lower colon. This is the most common pattern people notice, and it often comes from a benign cause like hemorrhoids or a small tear in the skin.
Black, tarry stool with a strong odor is a different signal. Blood that starts in the stomach or upper small intestine turns dark as it’s broken down by digestive enzymes during the long trip through the gut. This type of bleeding can be harder to recognize because it doesn’t look like blood at first glance.
Before assuming the worst, check what you’ve eaten or taken recently. Beets and foods with red coloring can make stool appear reddish. Iron supplements, bismuth-based antacids (the pink liquid many people keep in the medicine cabinet), black licorice, blueberries, and activated charcoal can all turn stool black without any bleeding involved.
Hemorrhoids and Anal Fissures
These two causes account for a large share of bright red rectal bleeding, especially in younger adults. They’re different problems, but both involve the tissue right at the anus.
Hemorrhoids are swollen veins around the anus. They often bleed during a bowel movement, leaving bright red blood on the paper or in the toilet. Most hemorrhoids don’t actually cause pain. You might notice itching, mild discomfort, or a visible lump near the anus, but painless bleeding is the hallmark.
Anal fissures are small tears in the skin lining the anus, usually caused by passing hard stool. They tend to hurt, sometimes sharply, during and after a bowel movement. You may also notice blood when you wipe, along with persistent itching or burning. Fissures are more painful than hemorrhoids because the torn skin can become irritated or infected.
Diverticular Bleeding
Diverticula are small pouches that form in weak spots of the colon wall, often where blood vessels pass through. They’re extremely common after age 40 and usually cause no symptoms at all. But if a small artery near one of these pouches erodes, it can bleed directly into the colon.
Diverticular bleeding is distinctive: it tends to be sudden, painless, and sometimes heavy. You may pass a significant amount of dark red or maroon-colored blood with little or no abdominal pain. Most episodes stop on their own, but they can recur. Heavy, continuous bleeding from diverticular disease is one reason to get to an emergency room quickly.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both involve an overactive immune system sending white blood cells into the lining of the digestive tract, where they release inflammatory chemicals that damage tissue and cause bleeding.
Ulcerative colitis is limited to the colon and almost always starts in the rectum. Bloody diarrhea is the classic symptom, often with sudden urgency and the feeling that you still need to go even after a bowel movement. Lower abdominal cramps, fatigue, and fever during serious flares are common.
Crohn’s disease can affect any part of the digestive tract, including the small intestine, and its diarrhea is often not bloody. When Crohn’s does cause bleeding, it’s typically because the colon is involved. The bigger red flags for Crohn’s are persistent belly pain, unintended weight loss, and malnutrition. Because inflammation can damage the part of the small intestine where nutrients are absorbed, the body may stop taking in enough from food.
If blood in your stool comes with ongoing diarrhea, cramping, weight loss, or fatigue that lasts weeks rather than days, inflammatory bowel disease is worth investigating.
Infections
Certain bacterial infections cause bloody diarrhea that comes on suddenly and resolves within days to a couple of weeks. The most well-known culprit is E. coli O157:H7, a strain that produces toxins damaging the intestinal lining. Symptoms typically begin three to four days after exposure, though they can start as early as one day or as late as a week later. Along with bloody loose stools, you may have severe stomach cramps and vomiting.
E. coli O157:H7 has also been linked to a complication that can cause kidney damage, particularly in young children. Other bacteria like Campylobacter and Salmonella can cause similar bloody diarrhea, usually tied to contaminated food or water. If bloody diarrhea hits suddenly and is accompanied by fever, it’s worth getting tested so the right infection can be identified.
Polyps and Colorectal Cancer
This is the possibility most people are worried about when they search this question. Colon polyps are small growths on the inner lining of the colon. Most polyps don’t cause symptoms and are found incidentally during screening. Some can bleed, though, and over time certain types can develop into cancer.
Colorectal cancer can cause rectal bleeding or blood mixed into the stool, but it rarely causes bleeding as its only symptom. Other signs include a persistent change in bowel habits (new constipation or diarrhea lasting more than a few days), unexplained weight loss, fatigue, and the feeling that your bowel doesn’t empty completely. Blood from a colon cancer may be invisible to the naked eye, which is why screening tests exist to detect hidden blood.
The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for average-risk adults, continuing through age 75. “Average risk” means no personal or family history of polyps, colorectal cancer, inflammatory bowel disease, or known genetic conditions like Lynch syndrome. If you have risk factors, screening typically starts earlier.
How Doctors Find the Source
Figuring out where the bleeding is coming from usually starts simply. A physical exam of the rectum can identify hemorrhoids or other issues near the surface. From there, your doctor may order one or more of the following:
- Fecal occult blood test: detects hidden blood in a stool sample, useful when bleeding isn’t visible
- Anoscopy or proctoscopy: a short scope to examine the anus and lower rectum
- Flexible sigmoidoscopy: examines the lower portion of the colon
- Colonoscopy: examines the entire colon and can remove polyps or take tissue samples during the same procedure
- Upper endoscopy: a scope passed through the mouth to check the stomach and upper small intestine, used when dark, tarry stool suggests the bleeding source is higher up
Which test you need depends on the color of the blood, your age, your symptoms, and your risk factors. A young person with obvious hemorrhoid symptoms may not need a colonoscopy. Someone over 45 with unexplained bleeding almost certainly will.
Signs That Need Immediate Attention
A small amount of bright red blood on the toilet paper after a hard bowel movement, with no other symptoms, is usually not an emergency. But certain combinations of symptoms signal that you’re losing enough blood to affect your body’s ability to function. Get emergency help if rectal bleeding comes with rapid or shallow breathing, dizziness or lightheadedness when you stand up, blurred vision, fainting, confusion, nausea, cold or clammy skin, or very low urine output. These are signs of shock from significant blood loss.
You should also get to an emergency room if the bleeding is continuous or heavy, or if it’s accompanied by severe abdominal pain or cramping. Even without those alarm signs, rectal bleeding that recurs over several weeks, or any amount of dark tarry stool, warrants a call to your doctor rather than a wait-and-see approach.