Blood in your stool is surprisingly common. About one in seven adults reports noticing it over any given 12-month period. The causes range from minor issues like hemorrhoids and small tears near the anus to more serious conditions like inflammatory bowel disease or colorectal cancer. The color of the blood is one of the most useful clues to where it’s coming from.
What the Color of the Blood Tells You
Bright red blood typically originates lower in the digestive tract, near the rectum or anus. You might see it on the toilet paper, coating the surface of your stool, or dripping into the bowl. Because the blood hasn’t traveled far, it stays red.
Black, tarry stools point to bleeding higher up, usually in the stomach or upper intestine. Blood that starts that high gets broken down by digestive enzymes as it moves through, which turns it dark and gives it a sticky, tar-like consistency. This type of bleeding isn’t always obvious at first glance, and it often has a distinct, strong odor. If your stools look black and tarry rather than simply dark from something you ate (iron supplements and bismuth medications can also darken stool), that warrants prompt medical attention.
Hemorrhoids and Anal Fissures
These two conditions are the most common reasons people notice blood after a bowel movement, and they’re both tied to the mechanics of passing stool.
Hemorrhoids are swollen veins in or around the rectum. They tend to cause a dull ache, itching, and sometimes a noticeable lump near the anus. The bleeding is usually painless or mildly uncomfortable, and you’ll see bright red blood on the tissue or in the bowl. Straining during bowel movements, sitting for long periods, pregnancy, and chronic constipation all increase your risk.
Anal fissures are small tears in the lining of the anus, usually caused by passing hard or large stools. The pain profile is very different from hemorrhoids. Fissures produce a sharp, tearing or burning sensation during a bowel movement that can linger for hours afterward. Where hemorrhoids itch, fissures burn or sting. Most fissures heal on their own within a few weeks with increased fiber and water intake to keep stools soft.
Diverticular Bleeding
Diverticula are small pouches that form in the wall of the colon, most often in the lower left side. They develop where the muscle layer of the colon wall is weakened by the blood vessels that pass through it. Most people with diverticula never have symptoms, but occasionally a blood vessel near one of these pouches erodes and bleeds.
When a diverticulum bleeds, it tends to be sudden and painless. You may pass a moderate to large amount of dark red or maroon-colored blood with little warning. The bleeding stops on its own in most cases, but the volume can be alarming enough to send people to the emergency room. Diverticular disease becomes more common with age and is closely linked to low-fiber diets.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both involve chronic inflammation of the digestive tract, but they show up differently when it comes to bleeding.
Ulcerative colitis is limited to the colon and always involves the rectum. Because inflammation sits so close to the exit, bloody diarrhea is one of its hallmark symptoms. People with ulcerative colitis often describe a sudden, urgent need to use the bathroom, cramping in the lower abdomen, and the persistent feeling that they still need to go even after finishing a bowel movement.
Crohn’s disease can affect any part of the digestive tract, including the small intestine. When Crohn’s involves the small intestine rather than the colon, diarrhea is often nonbloody. Belly pain and unintended weight loss are more prominent features. Bloody stools can still occur with Crohn’s, but they’re less consistently present than with ulcerative colitis.
Colorectal Polyps and Cancer
Polyps are small growths on the inner lining of the colon or rectum. Most are harmless, but some can develop into cancer over a period of years. Both polyps and colorectal cancer can bleed intermittently, often in amounts too small to see with the naked eye. When visible bleeding does occur, it may look like dark red blood mixed into the stool rather than bright red blood on the surface.
This is one reason routine screening matters even when you feel fine. The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for people at average risk. A colonoscopy, which examines the full length of the colon and rectum, is typically repeated every 10 years. During the procedure, a doctor can spot and remove polyps before they ever become cancerous. For people who prefer a less invasive option, the fecal immunochemical test (FIT) checks for hidden blood in the stool and can be done at home every year, though it can miss some polyps and cancers, and an abnormal result still requires a follow-up colonoscopy.
People with a family history of colorectal cancer or a personal history of inflammatory bowel disease may need to start screening earlier.
Less Common Causes
Stomach ulcers or erosion of the stomach lining, often from long-term use of anti-inflammatory pain relievers, can cause upper GI bleeding that shows up as black, tarry stools. Infections from certain bacteria or parasites can inflame the colon and cause bloody diarrhea that resolves once the infection clears. Abnormal blood vessels in the colon wall, called angiodysplasia, are another source of bleeding, particularly in older adults. Rarely, bleeding disorders or blood-thinning medications can make any of these conditions bleed more easily or more noticeably.
Signs That Require Emergency Care
Most rectal bleeding is minor and resolves on its own, but certain warning signs indicate you’re losing enough blood to need immediate help. Call emergency services or get to an emergency room if rectal bleeding is continuous or heavy, or if it comes with severe abdominal pain or cramping.
Signs of significant blood loss include:
- Dizziness or lightheadedness when standing up
- Rapid, shallow breathing
- Blurred vision or fainting
- Confusion
- Cold, clammy, or pale skin
- Very low urine output
- Nausea
These symptoms suggest your body is struggling to compensate for the volume of blood being lost, and waiting it out at home is not safe. Even without these dramatic signs, any new or unexplained bleeding that persists for more than a day or two, or that recurs over weeks, is worth getting evaluated. A single episode of bright red blood on the toilet paper after straining is far less concerning than repeated dark or mixed-in blood over time, but neither should be permanently ignored.