Blood in your stool can come from anywhere along your digestive tract, from your esophagus to your rectum. The cause ranges from something as common as hemorrhoids to something that needs urgent attention like an ulcer or colon polyps. The color of the blood is one of the most useful clues to where the bleeding is coming from.
What the Color of Blood Tells You
Bright red blood typically means the bleeding is coming from your lower digestive tract, specifically your colon, rectum, or anus. Because the blood hasn’t traveled far, it stays red and may coat the surface of your stool, drip into the toilet, or show up on toilet paper.
Black, tarry stool signals bleeding higher up, usually in your stomach or upper small intestine. The blood turns dark as it travels the full length of your digestive system and gets broken down along the way. This type of stool often has a sticky consistency and a distinct, strong smell. Some people also notice dark red blood mixed into their stool, which can indicate bleeding from either location depending on how fast it moves through.
Common Lower GI Causes
The most frequent reason people see bright red blood in their stool is hemorrhoids, which are swollen veins in the anus or lower rectum. They’re extremely common, especially during pregnancy or after straining with bowel movements, and the bleeding is usually painless and small in volume.
Anal fissures are another everyday cause. These are small tears in the thin tissue lining the anus, often from passing hard stool. Unlike hemorrhoids, fissures tend to cause a sharp, stinging pain during bowel movements along with a streak of bright red blood.
Diverticulosis, a condition where small pouches form in the colon wall, affects many adults as they age. About 10% of people with diverticulosis experience some bleeding. It’s usually temporary and painless, but it can produce a noticeable amount of fresh blood in your stool.
Colon polyps are small clumps of cells on the colon lining that can bleed intermittently. Most polyps are harmless, but some are cancerous or can become cancerous if left in place. Routine screening catches polyps before they become a problem, which is why the U.S. Preventive Services Task Force recommends colorectal cancer screening for adults starting at age 45.
Upper GI Causes
Peptic ulcers are among the most common causes of upper GI bleeding. These are open sores on the lining of your stomach or the first part of your small intestine. The two biggest risk factors for developing them are infection with a bacterium called H. pylori and regular use of pain relievers like aspirin, ibuprofen, or naproxen. Ulcers often cause a burning stomach pain between meals, though some bleed without much pain at all.
Gastritis, or inflammation of the stomach lining, can also lead to shallow breaks that bleed. The same culprits apply: H. pylori, pain relievers, blood thinners, and heavy alcohol use. Inflammation in the esophagus, often driven by chronic acid reflux, can produce ulcers there as well.
Less common but more serious causes include enlarged veins in the esophagus or stomach (most often related to liver disease) and tears in the lower esophagus from severe or prolonged vomiting. Tumors in the esophagus or stomach can also weaken the digestive lining and expose blood vessels.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are the two main forms of inflammatory bowel disease (IBD). In both conditions, the immune system mistakenly attacks the cells lining the digestive tract, creating chronic inflammation. Ulcerative colitis causes inflammation and open sores along the colon and rectum, while Crohn’s disease can affect any part of the digestive tract and penetrates deeper into the bowel wall.
Bloody stool is one of the hallmark symptoms of IBD, particularly ulcerative colitis. The bleeding comes directly from ulcerated tissue and is often accompanied by diarrhea, abdominal cramping, and urgency. Symptoms tend to flare and subside in cycles, so you might notice blood in your stool during some periods and not others.
Infections That Cause Bloody Stool
Certain bacterial infections can cause bloody diarrhea, usually after eating contaminated food or water. Bacteria like E. coli, Shigella, Salmonella, and Campylobacter invade the intestinal lining, creating ulcers and triggering an inflammatory response. Some strains of E. coli and Shigella produce toxins that cause more severe bleeding and, in rare cases, serious complications affecting the kidneys and nervous system.
Infectious bloody diarrhea typically comes on suddenly, lasts a few days, and is accompanied by fever, cramping, and nausea. Most cases resolve on their own, but heavy bleeding or high fever warrants medical attention.
How Medications Increase Bleeding Risk
Pain relievers like aspirin, ibuprofen, and naproxen raise the risk of GI bleeding through two mechanisms: they reduce your blood’s ability to clot, and they damage the protective mucus layer of your stomach lining. Over time, this combination can lead to ulcers, erosion, and bleeding that may be obvious or too small to see without testing. Prescription blood thinners carry a similar risk. One large study found that starting a pain reliever while already on a blood thinner increased the risk of significant bleeding by about 60%.
If you take these medications regularly and notice dark or bloody stool, that connection is worth flagging for your doctor. Even low-dose aspirin taken daily for heart health can cause enough stomach irritation to produce bleeding over months or years.
How Bleeding Is Detected
Sometimes bleeding is invisible. Slow, chronic blood loss can go unnoticed until it causes anemia, fatigue, or is picked up on a screening test. Two types of stool tests are commonly used to detect hidden blood. The newer version, called FIT, requires only one stool sample and no dietary changes beforehand. It detects about 76% of colorectal cancers, compared to roughly 39% for the older test, which requires samples from three consecutive bowel movements and dietary restrictions. Both tests are used as part of routine colorectal cancer screening between ages 45 and 75.
When blood is visible or a screening test comes back positive, a colonoscopy is typically the next step. This lets a doctor examine the full length of the colon, identify the source of bleeding, remove polyps, and take tissue samples if needed.
Signs That Need Immediate Attention
Most causes of blood in stool are not emergencies, but some are. Heavy or continuous rectal bleeding paired with severe abdominal pain warrants a trip to the emergency room. If you’re also experiencing rapid shallow breathing, dizziness when standing, blurred vision, fainting, confusion, nausea, or cold and clammy skin, these are signs of significant blood loss and you should call 911. Low urine output is another signal that your body isn’t getting enough blood flow to vital organs.
Even without those emergency signs, blood in your stool that persists for more than a day or two, or that keeps coming back, is worth getting evaluated. Chronic, low-level bleeding can sometimes alternate between black tarry stool and red blood depending on where and how fast the bleeding occurs.