Blood in your stool usually comes from somewhere along your digestive tract, and the color of the blood is the single best clue to where it’s coming from. Bright red blood typically points to a problem in the lower digestive tract (colon, rectum, or anus), while black, tarry, foul-smelling stool suggests bleeding higher up, in the stomach or upper intestine. Most causes are not life-threatening, but some require prompt attention.
What the Color Tells You
When blood originates high in the digestive tract, such as the stomach or the first part of the small intestine, digestive chemicals break it down during its long journey through the gut. That process turns the blood dark and sticky, producing a black, tar-like stool with a distinctly strong odor. The longer the blood has traveled, the darker and smellier it gets. This type of stool signals conditions like stomach ulcers, severe inflammation of the stomach lining, or ruptured veins in the esophagus.
Bright red blood, on the other hand, hasn’t been digested. It generally comes from the colon, rectum, or anus. You might see it on the toilet paper, coating the outside of the stool, or mixed into diarrhea. Small amounts of bright red blood often point to hemorrhoids, a fissure, or a polyp. Larger volumes can come from pouches in the colon wall (diverticula) or abnormal blood vessel clusters.
Hemorrhoids and Anal Fissures
These two conditions are the most common reasons people notice blood after wiping, and they’re often confused with each other. The key difference is pain. An anal fissure is a small tear in the lining of the anus that causes sharp, burning pain lasting minutes to hours after a bowel movement. The bleeding is usually a small streak of bright red blood on the toilet paper. Fissures are nearly invisible from the outside.
Hemorrhoids tend to bleed more noticeably, sometimes dripping into the toilet bowl or producing small clots, but the pain is milder: more of a dull ache or itch that comes and goes. External hemorrhoids often create a swollen lump you can feel near the anus. Both conditions are triggered or worsened by straining, hard stools, and prolonged sitting, and both usually improve with dietary fiber, adequate water, and softer stools.
Stomach Ulcers and Upper GI Causes
A bleeding ulcer in the stomach or the upper small intestine is the most common reason for black, tarry stool. Ulcers form when the protective lining of the stomach or intestine erodes, often due to a bacterial infection (H. pylori) or heavy use of over-the-counter pain relievers like ibuprofen and aspirin. Other upper-tract causes include severe inflammation of the stomach lining, tears in the esophagus from forceful vomiting, and swollen veins in the esophagus that can rupture (a condition linked to liver disease).
If your stool is black and sticky rather than simply dark from food like blueberries or iron supplements, that distinction matters. True tarry stool has a characteristic foul smell that’s hard to mistake. It warrants a same-day call to a healthcare provider because even slow upper GI bleeding can become significant over time.
Inflammatory Bowel Disease
Ulcerative colitis is an immune-driven disease that creates ulcers in the lining of the large intestine. Bloody stool is one of its hallmark symptoms, often accompanied by urgent diarrhea, crampy abdominal pain, mucus in the stool, fatigue, and weight loss. Many people also notice they need to use the bathroom at night, which is unusual with less serious causes of bleeding.
Crohn’s disease, the other major form of inflammatory bowel disease, can also cause bloody stools, though it tends to affect deeper layers of the intestinal wall and can occur anywhere from mouth to anus. Both conditions are chronic and tend to flare and remit. If you’re seeing blood alongside persistent diarrhea, weight loss, or fatigue that lasts more than a couple of weeks, those symptoms together paint a different picture than a one-time spot of blood on toilet paper.
Diverticular Bleeding
Diverticula are small, outward bulges that form in weakened areas of the colon wall, especially in people over 50. Most people with these pouches never know they have them. But when a blood vessel near one of these pouches ruptures, the result can be a sudden, painless, and sometimes alarming amount of bright red or maroon blood in the toilet. Diverticular bleeding is the most common cause of large-volume lower GI bleeding in older adults. It usually stops on its own, but heavy or repeated episodes need medical evaluation.
Infections That Cause Bloody Diarrhea
Food poisoning from bacteria like E. coli, Salmonella, and Campylobacter can cause bloody diarrhea, typically alongside high fever, cramping, and nausea. The bleeding happens because these pathogens damage the intestinal lining directly. This type of bloody stool comes on suddenly, often within a day or two of eating contaminated food, and usually resolves within a week. Persistent bloody diarrhea with fever that lasts beyond a few days, or signs of dehydration, calls for medical attention.
Blood Thinners and Medications
If you take a blood-thinning medication, your risk of GI bleeding is meaningfully higher. A large population study found that among patients on common oral anticoagulants, GI bleeding occurred at rates between roughly 3 and 9 cases per 100 person-years depending on the specific drug. Blood thinners don’t cause bleeding on their own, but they make it much harder for existing problems (a small ulcer, a polyp, a hemorrhoid) to stop bleeding naturally. If you’re on a blood thinner and notice blood in your stool, report it promptly rather than assuming it’s minor.
Over-the-counter pain relievers like ibuprofen, naproxen, and aspirin can also irritate the stomach lining enough to cause bleeding, particularly with regular use. This is one of the more preventable causes.
Colorectal Cancer and Polyps
This is the concern that brings most people to a search engine, so it’s worth being direct: colorectal cancer is a real but relatively uncommon cause of blood in the stool. Polyps (small growths on the colon wall) are more common and sometimes bleed intermittently. Most polyps are benign, but some can become cancerous over years if not removed. The blood from a polyp or tumor is often small in quantity and may not be visible to the naked eye.
The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for adults at average risk. “Average risk” means no prior polyps, no inflammatory bowel disease, and no family history of hereditary cancer syndromes like Lynch syndrome. People with those risk factors often need earlier or more frequent screening. The two most common screening tools are a stool-based test that detects hidden blood (the fecal immunochemical test, or FIT, which is about 74% sensitive for cancer) and colonoscopy, which allows direct visualization and removal of polyps in the same procedure.
Clues That Help You Gauge Severity
- A small amount of bright red blood on toilet paper after straining is the most common scenario and usually points to hemorrhoids or a fissure.
- Blood mixed into the stool or bloody diarrhea suggests a source deeper in the colon, such as colitis, an infection, or a polyp.
- Black, tarry, foul-smelling stool points to bleeding in the stomach or upper intestine and needs prompt evaluation.
- Large volumes of blood with lightheadedness or rapid heartbeat signal significant blood loss and require emergency care.
- Blood in the stool alongside unexplained weight loss, persistent changes in bowel habits, or fatigue warrants a thorough workup regardless of the amount of blood.
One episode of minor bright red bleeding in an otherwise healthy person under 45 is rarely an emergency. Recurrent bleeding, any amount of black tarry stool, or bleeding paired with other symptoms changes the calculation significantly.