Blood in your stool can come from dozens of sources, ranging from a minor anal fissure to a serious condition like colon cancer. The color and amount of blood offer the first clue about where the bleeding is happening. Bright red blood typically points to a problem in the lower digestive tract (the colon, rectum, or anus), while black, tarry stool suggests bleeding higher up, in the stomach or small intestine.
What the Color of Blood Tells You
Bright red blood on the toilet paper, on the surface of your stool, or in the bowl generally means bleeding from the colon, rectum, or anus. Small amounts often come from hemorrhoids, fissures, or polyps. Larger volumes of bright red blood can signal diverticular bleeding or a vascular malformation in the colon.
Black, sticky, tarry stool with a strong odor is a different situation entirely. It takes only about 100 to 200 cc of blood (roughly half a cup) higher in the digestive tract to produce this appearance. Digestive enzymes break down the blood as it moves through the intestines, turning it dark and changing its consistency. This type of bleeding usually originates above the colon, in the stomach or upper small intestine, and can indicate an ulcer, gastritis, or another upper GI problem.
Maroon-colored stool falls somewhere in between and may indicate bleeding from the small intestine or the right side of the colon.
Foods and Medications That Mimic Blood
Before assuming the worst, consider what you’ve recently eaten or taken. Beets and foods with red coloring can make stool appear reddish. On the other end, black licorice, blueberries, iron supplements, activated charcoal, and bismuth-based medications like Pepto-Bismol can all turn stool black. These color changes are harmless, but if you haven’t consumed any of these and your stool looks bloody or tarry, the color likely reflects actual bleeding.
Hemorrhoids and Anal Fissures
These are the two most common causes of bright red blood in the stool, and they share similar triggers. Straining during bowel movements, constipation, and passing hard stool can cause both. The key difference is pain. Hemorrhoids don’t always hurt. You might notice blood on the paper or dripping into the bowl without any discomfort at all. Fissures, on the other hand, cause pain about 90% of the time. People describe it as sharp, tearing, or burning, and it’s worst during a bowel movement but can linger for minutes to hours afterward.
Hemorrhoid pain, when it does occur, tends to be more constant. Fissure pain comes in episodes, spiking with each bowel movement and then easing. Both conditions usually improve with dietary fiber, adequate water intake, and stool softeners, though persistent cases may need further treatment.
Diverticular Bleeding
Diverticulosis, where small pouches form in the wall of the colon, is extremely common as people age. About 10% of people with these pouches experience some bleeding. It happens when hard stool passing through a pouch erodes or stretches a blood vessel until it breaks. The hallmark of diverticular bleeding is that it’s usually painless but can produce a startling amount of fresh blood in the stool. Most episodes stop on their own, but heavy or repeated bleeding needs medical evaluation.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease cause chronic inflammation in the digestive tract, and bloody stool is one of their defining symptoms. Ulcerative colitis specifically inflames the lining of the colon, producing diarrhea that often contains blood or pus along with rectal pain. Unlike a one-time episode of bleeding, inflammatory bowel disease comes with a broader pattern: fatigue, fever, weight loss, and bouts of diarrhea that persist for weeks.
Over time, ulcerative colitis can lead to complications beyond the gut, including bone loss, skin inflammation, joint problems, eye inflammation, and an increased risk of colon cancer. If you’re experiencing bloody diarrhea alongside unexplained fatigue or weight loss, inflammatory bowel disease is one of the conditions your doctor will want to rule out.
Infections That Cause Bloody Stool
Several bacterial infections can trigger bloody diarrhea. Certain strains of E. coli, particularly Shiga toxin-producing E. coli (STEC), cause bloody diarrhea with severe stomach cramps and vomiting. Other strains cause watery diarrhea that can become bloody, sometimes with fever. These infections are typically picked up from contaminated food or water and usually resolve within a week, though some can become serious, especially in young children and older adults.
Bloody diarrhea that comes on suddenly with fever and cramping after eating undercooked meat, unwashed produce, or food from a questionable source is worth investigating for a bacterial infection.
Colorectal Cancer
This is the possibility most people are worried about when they search for bloody stool, and it’s worth addressing directly. Colorectal cancer can cause blood in the stool, but it’s far less common than hemorrhoids or fissures as the explanation. The blood from a tumor is sometimes visible, sometimes not. When the amount is too small to see, it’s called occult (hidden) blood, and it can only be detected with a lab test.
The U.S. Preventive Services Task Force recommends that most adults begin routine colorectal cancer screening at age 45, continuing through age 75. Screening is designed to catch cancer or precancerous polyps before symptoms ever appear, which is why it matters even if you feel fine.
How Doctors Find the Source
If you report blood in your stool, your doctor will likely start with one of a few approaches depending on your age, symptoms, and risk factors.
- Stool tests: A fecal immunochemical test (FIT) or fecal occult blood test (FOBT) checks a stool sample for hidden blood. These are simple and noninvasive, but they can miss some polyps and cancers, and they sometimes flag blood when nothing serious is wrong (a false positive). If blood is detected, further testing is needed to find the source.
- Colonoscopy: This is the most thorough option. It allows a doctor to see the entire colon and rectum, remove polyps, and take tissue samples during the same procedure. It can miss some very small polyps, but it remains the most sensitive screening tool available for colorectal cancer.
- Stool DNA test: This newer option checks for both hidden blood and DNA changes in cells that might indicate cancer or precancerous growths. It’s less sensitive than colonoscopy at detecting precancerous polyps, and abnormal results still require a colonoscopy to confirm.
When Bloody Stool Is an Emergency
Most causes of blood in the stool are not emergencies, but some are. If you notice blood in your stool and feel faint, dizzy, or lightheaded, that combination suggests you may be losing a significant amount of blood. A rapid heart rate, clammy skin, or feeling like you might pass out are signs of the same problem. This is a situation that calls for emergency care immediately, not a scheduled appointment. Large-volume bleeding from conditions like diverticulosis or a bleeding ulcer can escalate quickly, and waiting it out is not safe.