Bloody stool has a wide range of causes, from hemorrhoids and small tears near the anus to infections, inflammatory bowel disease, and, less commonly, colorectal cancer. Hemorrhoids are the single most common cause. The color 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 (the colon, rectum, or anus), while black, tarry stools point to bleeding higher up, such as the stomach or small intestine.
What the Color of Blood Tells You
Bright red blood on toilet paper, in the bowl, or mixed into stool means the bleeding source is relatively close to the exit. Blood hasn’t had time to be broken down by digestive enzymes, so it keeps its red color. This is most often linked to hemorrhoids, anal fissures, or conditions affecting the colon.
Black, tarry, sticky stools look and smell different from normal bowel movements. The dark color comes from hemoglobin, a protein in blood, being chemically altered as it passes through the stomach and small intestine. This type of bleeding can signal ulcers, inflammation, or growths in the upper digestive tract and generally warrants prompt medical attention.
It’s also worth knowing that certain foods and supplements can mimic the appearance of blood. Beets, dragon fruit, blackberries, and rhubarb can turn stool red or pink. Iron supplements and bismuth (the active ingredient in some stomach remedies) can make stool look black. If you’ve consumed any of these within the past 48 hours, that may explain the color change before anything more concerning does.
Hemorrhoids and Anal Fissures
Hemorrhoids are swollen veins in the rectum or anus. They’re the most common cause of blood in stool and are usually not serious. Straining during bowel movements, sitting for long periods, and chronic constipation all increase the risk. The blood is typically bright red and shows up on toilet paper or drips into the bowl. Internal hemorrhoids are painless; external ones can itch, swell, and hurt.
Anal fissures are small tears in the lining of the anal canal, also frequently caused by straining or passing hard stools. They produce bright red blood too, but tend to come with a sharp, stinging pain during and after a bowel movement. Most fissures heal on their own within a few weeks with softer stools and good hydration.
Bacterial Infections
Several types of foodborne or waterborne bacteria can inflame the colon and cause bloody diarrhea. The mechanism is straightforward: the bacteria damage the lining of the intestine, producing inflammation that leads to blood and sometimes pus in the stool.
The most common culprits include Campylobacter (often from undercooked poultry), Shigella (spread through contaminated food or water), and certain strains of E. coli, particularly the type responsible for hemorrhagic colitis, which causes bloody diarrhea along with abdominal pain, nausea, and vomiting. Yersinia enterocolitica produces bloody stools in roughly 20 to 46 percent of cases, depending on the strain and how many organisms were ingested. C. difficile, which frequently develops after antibiotic use, can cause severe diarrhea that may turn bloody, along with fever and intense abdominal pain.
Most bacterial gastroenteritis episodes resolve within a week, but infections involving high fever, severe cramping, or large amounts of blood need medical evaluation. Some, like certain E. coli strains, can lead to serious complications affecting the kidneys.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are the two main forms of inflammatory bowel disease (IBD). They cause chronic inflammation in the digestive tract, and bloody stool is one of their hallmark symptoms, particularly in ulcerative colitis, which affects the colon and rectum. People with ulcerative colitis often notice blood mixed with mucus in their stool, along with urgency and frequent trips to the bathroom.
Crohn’s disease behaves differently. It can affect any part of the digestive tract and creates deep ulcers that grow outward through the intestinal wall. Diarrhea from Crohn’s is usually not bloody, though it can be. Crohn’s also commonly causes anal fissures from chronic inflammation, which can produce visible bleeding. Both conditions involve periods of flare-ups and remission, and both are diagnosed through a combination of colonoscopy, imaging, and lab work.
Diverticular Bleeding
Diverticular bleeding is the most common cause of sudden, large-volume bright red bleeding from the lower digestive tract in adults. Diverticula are small pouches that form in the wall of the colon, usually after age 40. Most people with diverticula never have problems, but occasionally a blood vessel near one of these pouches erodes and bleeds.
What makes diverticular bleeding distinctive is how it starts: suddenly and painlessly, often producing a large amount of blood. It’s alarming, and most people who experience it end up in the hospital. The good news is that in the majority of cases, the bleeding stops on its own. The bad news is that it can recur.
Colorectal Cancer and Polyps
Blood in the stool can be an early sign of colorectal cancer or precancerous polyps. Polyps are growths on the inner lining of the colon that can slowly bleed, sometimes in amounts too small to see with the naked eye. This is called occult (hidden) bleeding, and it’s the reason screening tests exist. Over time, some polyps develop into cancer.
Colorectal cancer screening is recommended for all adults starting at age 45, continuing through age 75. Several options are available. The simplest are stool-based tests: the fecal immunochemical test (FIT) and the guaiac-based fecal occult blood test, both done yearly, or a FIT-DNA test done every three years. These check for hidden blood or abnormal DNA in stool samples you collect at home. Colonoscopy, the most thorough option, examines the entire colon and is repeated every 10 years for people at average risk. Other options include flexible sigmoidoscopy every five years and CT colonography (virtual colonoscopy) every five years.
Bloody stool from colorectal cancer doesn’t look a specific way. It can be bright red, dark red, or invisible. That’s why screening matters regardless of symptoms, especially if you have a family history of colorectal cancer or polyps.
Medications That Increase Bleeding Risk
Certain medications make bleeding anywhere in the digestive tract more likely. Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin, ibuprofen, and naproxen can irritate the stomach lining and, over time, cause ulcers that bleed. Blood thinners reduce the body’s ability to form clots, which means any small source of bleeding, from a hemorrhoid to a polyp, may produce more visible blood than it otherwise would. If you take any of these regularly and notice blood in your stool, that’s important information to share with your doctor.
How Doctors Find the Source
Diagnosing the cause of bloody stool usually starts with a conversation about your symptoms, medications, and family history, followed by a physical exam that may include a digital rectal exam. From there, the workup depends on what the doctor suspects.
Lab tests come first in most cases. Stool tests can confirm the presence of hidden blood. Blood tests reveal whether you’ve lost enough blood to become anemic and help gauge severity. If the source isn’t obvious, endoscopy is the next step. An upper endoscopy examines the esophagus, stomach, and the first part of the small intestine. A colonoscopy examines the rectum and entire colon. For bleeding in the small intestine, which is harder to reach, capsule endoscopy is an option: you swallow a pill-sized camera that takes thousands of images as it moves through your digestive tract.
When bleeding is severe or hard to locate, imaging tests like CT angiography or radionuclide scanning can pinpoint where blood is escaping. Angiography, which uses dye injected into blood vessels, is typically reserved for cases where other methods haven’t identified the source. Surgery to locate the bleeding is a last resort, used only when bleeding is severe and won’t stop.
Signs That Need Emergency Attention
Most causes of bloody stool are not emergencies, but some are. Seek emergency care 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 significant blood loss, and they mean the body is struggling to maintain normal circulation. Large-volume bleeding that doesn’t slow down on its own also warrants an emergency visit, regardless of other symptoms.