Blood in your stool has a wide range of causes, from something as minor as a small tear in the skin around your anus to something that needs prompt medical attention like an ulcer or inflammatory bowel disease. The color of the blood is one of the most useful clues: bright red blood typically comes from the lower digestive tract (colon, rectum, or anus), while black, tarry stools usually signal bleeding higher up, like the stomach or upper intestine. It takes roughly 100 to 200 mL of blood in the upper digestive tract to turn stool black.
What the Color Tells You
Bright red blood on the toilet paper, in the bowl, or mixed into your stool points to bleeding somewhere in the colon, rectum, or anus. This is the most common presentation and is often caused by conditions close to the exit point, like hemorrhoids or fissures. In some cases, though, a fast, heavy bleed higher up in the digestive tract can also produce bright red blood because the blood moves through quickly enough that it doesn’t darken.
Black, sticky, tar-like stool has a distinctive look and smell. It forms when blood sits in the digestive tract long enough for stomach acid to break down the red pigment in hemoglobin, turning it dark brown or black. This type of stool generally means the bleeding source is in the stomach, esophagus, or upper small intestine.
Dark red or maroon-colored stool falls in between and can come from the small intestine or the right side of the colon.
The Most Common Causes
Hemorrhoids are the leading cause of bright red rectal bleeding. They’re swollen veins around the anus that can bleed when you strain during a bowel movement. Most hemorrhoids don’t actually cause pain. You might notice blood on the paper or dripping into the bowl, and sometimes feel a soft lump near the opening. They’re extremely common, especially during pregnancy, with chronic constipation, or after long periods of sitting.
Anal fissures are small tears in the skin lining the anus, and they tend to hurt noticeably, especially during and after a bowel movement. The pain distinguishes them from hemorrhoids. You may also feel burning or itching that lingers. Fissures usually result from passing a large or hard stool.
Diverticular bleeding is another frequent cause, particularly in people over 40. Small pouches called diverticula form in weak spots of the colon wall over time. Blood vessels stretch across these pouches and can rupture. The hallmark of diverticular bleeding is a sudden, painless episode of significant rectal bleeding. It often stops on its own, but the volume of blood can be alarming.
Infections That Cause Bloody Diarrhea
Several types of food-borne bacteria can inflame the intestinal lining enough to cause bloody diarrhea, usually alongside fever and cramping. The most common culprits are E. coli, Salmonella, Shigella, and Campylobacter. If you’ve recently eaten undercooked meat, traveled internationally, or been around someone with similar symptoms, an infection is a strong possibility. Bacterial infections typically come on within one to a few days of exposure and resolve within a week, though some need treatment.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both cause recurrent episodes of bloody diarrhea, abdominal pain, fatigue, and weight loss. Ulcerative colitis affects only the colon and rectum and almost always involves bloody diarrhea with cramping. Crohn’s disease can affect any part of the digestive tract and tends to cause abdominal pain one to two hours after eating, with diarrhea that may or may not be bloody.
If you’re having persistent or recurring bloody stools alongside ongoing cramping, urgency, unexplained weight loss, or fatigue, inflammatory bowel disease is worth investigating. These are chronic conditions, so the bleeding tends to come and go over weeks or months rather than appearing as a single isolated episode.
Medications That Increase Bleeding Risk
Common over-the-counter pain relievers like ibuprofen, naproxen, and aspirin can damage the stomach lining and cause bleeding. These drugs work by blocking protective chemicals that keep the stomach’s inner lining intact. They reduce the mucus barrier, increase acid exposure, and decrease blood flow to the stomach wall, all of which makes ulcers more likely. The risk increases with higher doses and longer use, and it stays elevated the entire time you’re taking them.
Aspirin is particularly notable. One analysis found it was responsible for 18% of drug-related hospital admissions, and GI bleeding accounted for 72% of those cases. If you’re taking aspirin alongside another anti-inflammatory, the combined risk of bleeding is higher than either drug alone. Blood-thinning medications (anticoagulants and antiplatelet drugs) also increase the chance that any existing irritation in the digestive tract will bleed.
Foods That Mimic Blood in Stool
Before assuming the worst, consider what you’ve eaten recently. Several foods and supplements can make stool look bloody when no actual bleeding is occurring.
- Red-colored stools: Beets, red gelatin, red fruit punch, red licorice, and red-dyed snack foods can all produce stool that looks like it contains fresh blood.
- Black-colored stools: Iron supplements, bismuth-containing stomach medicines (like Pepto-Bismol), activated charcoal, blueberries, and large amounts of dark leafy greens like spinach or kale can turn stool black and mimic the appearance of upper GI bleeding.
If you recently ate any of these and feel fine otherwise, that may be the explanation. The color change typically resolves within a day or two of stopping the food or supplement.
Colorectal Cancer and Polyps
Colorectal cancer is a less common but serious cause of blood in stool, and it’s the reason routine screening matters. Polyps, which are small growths on the colon lining, can bleed intermittently. Most polyps are harmless, but some develop into cancer over time. The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for people at average risk. Screening is strongly recommended for everyone between 50 and 75.
One of the simplest screening tools is a fecal immunochemical test, or FIT, which detects tiny amounts of blood in your stool from the lower intestines. Unlike older stool tests, FIT isn’t affected by what you eat or what supplements you take, which reduces false positives. Any abnormal stool test result should be followed up with a colonoscopy, where a doctor examines the full length of the colon using a flexible camera.
Signs That Bleeding Is Serious
Most causes of bloody stool are not emergencies, but some are. Pay close attention if you experience any of the following alongside blood in your stool: feeling faint, lightheaded, or dizzy; a rapid heartbeat; confusion; or cold, clammy skin. These are signs of significant blood loss and possible shock. Large-volume bleeding, whether bright red or dark, that doesn’t stop or comes with these symptoms needs immediate medical evaluation.
Persistent bleeding over days or weeks, even in small amounts, also warrants investigation. Chronic low-level blood loss can lead to anemia, which causes fatigue, weakness, and shortness of breath that creeps up gradually. Unexplained weight loss, changes in stool caliber (narrow or pencil-thin stools), or new onset of symptoms after age 45 are additional reasons to get evaluated promptly.