Blood in the stool has a wide range of causes, from hemorrhoids and small tears near the anus to ulcers, inflammatory bowel disease, and colorectal cancer. The color and appearance of the blood is one of the most useful clues to where the bleeding originates. Bright red blood typically comes from the lower digestive tract (colon, rectum, or anus), while black, tarry stools usually signal bleeding higher up, in the stomach or esophagus.
What the Color of the Blood Tells You
Bright red blood on toilet paper, in the bowl, or mixed into the stool points to a source in the colon, rectum, or anus. The medical term for this is hematochezia. Because the blood hasn’t traveled far, it stays red.
Black, sticky, tar-like stools point to bleeding in the stomach or upper small intestine. As blood moves through the digestive tract, stomach acid and enzymes break it down, turning it dark. This type of stool, called melena, often has a distinct foul smell that’s different from a normal bowel movement. The most common causes are stomach ulcers and inflammation of the esophagus or stomach lining.
Sometimes blood is invisible to the naked eye. Slow, chronic bleeding from polyps or other sources can go undetected for months, gradually depleting your iron stores. The result is iron deficiency anemia, which shows up as persistent tiredness and shortness of breath long before you notice any change in your stool.
Hemorrhoids and Anal Fissures
These are the most common reasons people see bright red blood after a bowel movement, and they’re almost always tied to straining. Hemorrhoids are swollen veins in the rectum or anus. They can be internal (inside the rectum, where you may not feel them) or external (around the anal opening, where they can itch or hurt). Bleeding from hemorrhoids is typically painless, showing up as bright red streaks on toilet paper or dripping into the bowl.
An anal fissure is a small tear in the lining of the anal canal, also often caused by straining or passing hard stool. Unlike hemorrhoids, fissures tend to cause a sharp, stinging pain during and after a bowel movement. Both conditions usually heal on their own with softer stools, more fiber, and adequate water intake, though persistent or heavy bleeding warrants further evaluation.
Diverticular Bleeding
Diverticulosis, the most common cause of significant bright red rectal bleeding, happens when small pouches form in the colon wall. These pouches are extremely common in adults over 50 and usually cause no symptoms at all. Occasionally, though, a blood vessel near one of these pouches erodes and bleeds. The bleeding can be sudden and heavy, filling the toilet bowl with bright or dark red blood. Most diverticular bleeds stop on their own, but the volume of blood can be alarming enough to prompt an emergency room visit.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are the two main forms of inflammatory bowel disease (IBD). Both involve chronic inflammation of the digestive tract and can cause blood in the stool, though they do it differently.
Ulcerative colitis is limited to the colon and rectum. The immune system attacks the lining of the large intestine, creating ulcers that bleed. Bloody diarrhea, often with mucus, is one of its hallmark symptoms. The exact trigger isn’t fully understood, but it appears to involve an immune system that mistakenly targets the digestive tract, possibly set off by an imbalance in gut bacteria. Genetics play a role too: several genetic markers have been linked to the condition, and it tends to run in families.
Crohn’s disease can affect any part of the gastrointestinal system, from the mouth to the anus, and often involves deeper layers of the bowel wall. Bleeding is less consistently visible than in ulcerative colitis, but it still occurs, particularly when the colon is involved. Both conditions are diagnosed through a combination of blood tests, stool samples, and colonoscopy with tissue biopsy.
Colon Polyps and Colorectal Cancer
Polyps are growths on the inner lining of the colon. Most are harmless and cause no symptoms, which is exactly why routine screening matters. When polyps do bleed, the blood may show up as red streaks in the stool or cause stools to appear black. More often, the bleeding is so slow it’s invisible, only detectable through a stool test or when anemia shows up on blood work.
Some polyps, if left in place for years, can develop into colorectal cancer. A large polyp or tumor can also partially block the bowel, causing cramping and abdominal pain. The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for average-risk adults, continuing through age 75. Screening catches polyps before they become cancerous and detects cancers early, when they’re most treatable.
Infections That Cause Bloody Stool
Certain bacterial infections attack the lining of the intestine and cause bloody diarrhea. In the United States, the most significant culprit is a strain of E. coli called O157:H7, which produces toxins that damage the colon wall. It’s typically picked up from undercooked ground beef, contaminated produce, or unpasteurized dairy. In some people, particularly young children and older adults, this strain can trigger a serious complication that damages the kidneys.
Other bacteria that cause bloody diarrhea include Salmonella, Shigella, and Campylobacter. These infections usually come on suddenly with cramps, fever, and frequent watery or bloody stools. Most resolve within a week, though severe cases may need medical treatment to prevent dehydration or complications.
Stomach Ulcers and Upper GI Bleeding
Stomach ulcers and inflammation of the stomach or esophagus are the most common causes of black, tarry stools. The ulcers themselves are open sores in the stomach lining, most often caused by the bacterium H. pylori or by regular use of pain relievers like ibuprofen, aspirin, and naproxen.
The risk of bleeding from these medications is dose-dependent: higher doses and longer use increase the danger, and the risk stays linear over time rather than leveling off. Taking multiple pain relievers at once compounds the problem. Guidelines recommend using the lowest effective dose for the shortest period possible. A Mallory-Weiss tear, a small rip at the junction of the stomach and esophagus caused by prolonged vomiting or retching, is another upper GI source that produces dark or bloody stools.
Vascular Lesions
Sometimes the blood vessels in the wall of the intestine themselves are the problem. Angiodysplasia refers to abnormal, fragile blood vessels that can develop in the colon, particularly in older adults. These lesions may bleed intermittently, making them tricky to diagnose because the bleeding comes and goes. They’re more common in people with chronic kidney disease or heart disease and can be worsened by blood thinners or regular NSAID use.
Foods and Medications That Mimic Blood
Not every color change means bleeding. Beets and foods with red coloring can make stools appear reddish, closely mimicking the look of blood. On the dark side, iron supplements, activated charcoal, black licorice, blueberries, blood sausage, and bismuth-containing medications like Pepto-Bismol can all turn stools black. These changes are harmless, but if you’re not sure whether the color is from something you ate or from actual blood, a simple stool test can settle the question.
How Bleeding Is Evaluated
When you report blood in your stool, the evaluation depends on your age, symptoms, and what the blood looks like. A stool sample can detect hidden blood and check for signs of infection or inflammation. Blood tests check for anemia and infection markers. For visible or persistent bleeding, a colonoscopy allows direct visualization of the entire colon and the ability to take tissue samples, remove polyps, or treat a bleeding site in the same procedure. If the colon is too inflamed for a full colonoscopy, a shorter exam that reaches only the rectum and lower colon may be used instead. CT scans or MRIs can provide additional detail about the extent and location of inflammation or other abnormalities.