Blood in your poop usually comes from something minor and treatable, like hemorrhoids or a small tear near the anus. But it can also signal conditions that need medical attention, from infections to inflammatory bowel disease to, less commonly, cancer. What the blood looks like, where it shows up, and what other symptoms come with it all help narrow down the cause.
What the Color Tells You
The color of the blood is one of the most useful clues. Bright red blood typically means the bleeding is happening near the end of your digestive tract: the rectum, anus, or lower colon. You might see it on the toilet paper, in the bowl, or coating the surface of your stool. This kind of bleeding is extremely common and, in most cases, not dangerous.
Dark, tarry, almost black stool points to bleeding higher up in your digestive system, usually the stomach or the upper part of your small intestine. As blood travels through your gut, digestive chemicals break it down and change its color and texture. By the time it reaches the other end, it looks black and sticky rather than red. This type of stool has a distinctly foul smell that’s different from normal bowel movements. Dark stool from upper GI bleeding is generally considered more urgent than a streak of bright red on the toilet paper, because it often involves conditions like stomach ulcers or severe inflammation of the stomach lining.
That said, not every color change means bleeding. Beets and foods with red dye can make stool look reddish. Iron supplements, bismuth-based stomach remedies like Pepto-Bismol, black licorice, blueberries, and activated charcoal can all turn stool black. If you recently consumed any of these, that’s likely the explanation.
Common Causes of Bright Red Blood
Hemorrhoids are the most frequent culprit. These are swollen blood vessels in or around the anus that can bleed when you strain during a bowel movement. The blood is usually bright red, painless, and shows up on the toilet paper or drips into the bowl. Hemorrhoids are so common that many people experience them at some point, especially during pregnancy or periods of chronic constipation.
Anal fissures, small tears in the lining of the anus, are another everyday cause. They typically happen from passing a hard or large stool and produce a sharp, stinging pain during bowel movements along with a small amount of bright red blood. Most fissures heal on their own within a few weeks with softer stools and good hygiene.
Diverticular bleeding is more common in older adults. Diverticula are small pouches that form along the walls of the colon. They’re usually harmless, but a blood vessel inside one can rupture and cause sudden, painless bleeding that can sometimes be heavy. This type of bleeding often stops on its own, though it occasionally requires treatment during a colonoscopy to seal the broken vessel.
Infections That Cause Bloody Stool
Certain bacterial infections can inflame the lining of the colon enough to cause bloody diarrhea. The strain E. coli O157:H7 is the most common cause of hemorrhagic colitis in the United States. It typically starts with severe abdominal cramps and watery diarrhea that can turn bloody within 24 hours. Salmonella and Campylobacter infections can also produce blood in the stool, usually alongside fever, nausea, and cramping. These infections are most often linked to undercooked meat, contaminated water, or improperly handled food, and they generally resolve within a week, though some cases need medical treatment.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are chronic conditions where the immune system attacks the lining of the digestive tract. Both typically cause diarrhea (often bloody), abdominal pain, fatigue, and weight loss. Ulcerative colitis is particularly associated with bloody diarrhea because it affects the colon and rectum directly. Crohn’s disease can affect any part of the digestive tract, so bleeding is common but not always the main symptom.
If you’re seeing blood in your stool repeatedly over weeks, especially alongside loose stools, urgency, mucus, or unexplained weight loss, inflammatory bowel disease is one of the possibilities worth investigating. These conditions don’t go away on their own, but they can be managed effectively with ongoing treatment.
Upper GI Causes of Dark or Black Stool
Peptic ulcers are open sores in the stomach lining or the upper small intestine. They’re one of the most common causes of upper GI bleeding. If an ulcer erodes into a blood vessel, the resulting blood gets digested as it moves through your system, producing that characteristic black, tarry stool.
Other upper GI sources include severe inflammation of the stomach lining (often from heavy alcohol use or prolonged use of anti-inflammatory painkillers), tears in the esophagus from violent vomiting, and swollen veins in the esophagus or stomach that can rupture. Cancers of the stomach, esophagus, or pancreas can also cause slow, ongoing bleeding that turns stool dark, though these are far less common than ulcers or inflammation.
How Likely Is It to Be Cancer?
This is the question behind the question for most people searching this topic, so here are the numbers. A study tracking patients aged 45 and older in primary care found that among those who developed a new episode of rectal bleeding, about 5.7% turned out to have colorectal cancer. Roughly one in 10 had some form of abnormal growth, including non-cancerous polyps. The risk increased with age: among people 65 to 74, about 9.5% of those with new rectal bleeding had a cancerous or precancerous finding, compared to about 3.9% among those 45 to 54.
So while cancer is a real possibility, it’s far from the most likely explanation. The vast majority of rectal bleeding comes from benign causes. Still, the risk is high enough that new or persistent bleeding, particularly in adults over 45, warrants investigation rather than assumptions.
How Hidden Blood Is Detected
Sometimes blood in the stool isn’t visible at all. Slow, low-volume bleeding can go completely unnoticed until it’s picked up by a screening test. The fecal immunochemical test (FIT) is the most widely used option. It detects tiny amounts of blood in a stool sample and is about 80% accurate at catching colorectal cancer when used once. For advanced precancerous growths, the sensitivity is lower, around 20% to 30% with a single test, which is why repeating FIT annually is standard practice for ongoing screening. FIT is more accurate than the older guaiac-based stool tests and doesn’t require dietary restrictions beforehand.
Signs That Need Immediate Attention
A small amount of bright red blood on the toilet paper after straining is rarely an emergency. But certain combinations of symptoms call for urgent care. Heavy or continuous rectal bleeding, especially if it doesn’t slow down, warrants a trip to the emergency room. The same goes for bleeding paired with severe abdominal pain or cramping.
If rectal bleeding comes with signs of significant blood loss, call 911. Those signs include rapid or shallow breathing, dizziness or lightheadedness when you stand, blurred vision, fainting, confusion, nausea, cold or clammy skin, and reduced urine output. These symptoms suggest your body is losing blood faster than it can compensate, and that requires emergency treatment regardless of the underlying cause.