Blood in your stool is most commonly caused by hemorrhoids or anal fissures, both of which are benign and treatable. But the color, amount, and accompanying symptoms can point to a range of other causes, from infections and inflammatory conditions to polyps and, less often, cancer. The color of the blood is one of the most useful clues to where the bleeding is coming from.
What Blood Color Tells You
Bright red blood typically originates from the lower digestive tract: the colon, rectum, or anus. You might see it on the toilet paper, on the surface of your stool, or dripping into the bowl. The most common sources are hemorrhoids, anal fissures, and diverticular disease.
Dark red or maroon blood usually comes from higher in the colon, where conditions like inflammatory bowel disease or certain infections can damage the intestinal lining. Black, tarry stools signal bleeding even further up, in the stomach or esophagus. Blood that travels that distance gets digested along the way, which turns it dark and gives the stool a sticky, tar-like consistency. The most common causes of black tarry stools are stomach ulcers and inflammation of the esophagus or stomach lining.
Hemorrhoids and Anal Fissures
These two conditions account for the majority of bright red rectal bleeding, and telling them apart is fairly straightforward. Hemorrhoids are swollen blood vessels in and around the anus. Internal hemorrhoids are usually painless but bleed easily, often leaving bright red streaks on toilet paper or in the toilet bowl. External hemorrhoids can be painful, especially if a blood clot forms inside one, creating a hard, tender lump.
Anal fissures are small tears in the lining of the anus, usually caused by passing hard or large stools. The hallmark difference is pain: fissures cause a sharp, burning sensation during and after a bowel movement. The blood looks similar (bright red, on the toilet paper or stool surface), but the pain is hard to miss. Both conditions typically improve with more fiber, more water, and softer stools.
Diverticular Bleeding
Diverticulosis, where small pouches form in the wall of the colon, is extremely common as people age. Among those with diverticulosis, 15 to 20 percent will experience bleeding at some point. The bleeding is often sudden and painless, producing a noticeable amount of bright red or maroon blood. It can look alarming, but 70 to 90 percent of diverticular bleeds stop on their own without any intervention. Still, a significant or sudden bleed warrants medical evaluation, because the volume of blood loss can occasionally be substantial.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both involve chronic inflammation of the digestive tract, and both can cause blood in the stool. In ulcerative colitis, the inflammation targets the lining of the rectum and large intestine, and visible blood in the stool is one of the defining symptoms. Ulcers form directly in the intestinal lining and bleed as stool passes over them.
Crohn’s disease causes bleeding less frequently. It depends on where the inflammation is located. When Crohn’s affects the colon or rectum, blood in the stool is more likely. When it affects the small intestine, bleeding is less visible and may only show up on lab tests. Both conditions also come with other symptoms like persistent diarrhea, abdominal cramping, fatigue, and unintended weight loss, so blood in the stool rarely appears in isolation.
Infections That Cause Bloody Diarrhea
Several types of bacteria can cause bloody diarrhea, usually after consuming contaminated food or water. The most common culprits in developed countries are Campylobacter, Salmonella, Shigella, and a particularly dangerous strain of E. coli known as O157:H7. These infections typically come on suddenly with cramping, watery diarrhea that turns bloody, and sometimes fever.
E. coli O157:H7 deserves special attention because it can trigger a serious complication called hemolytic uremic syndrome, which damages the kidneys. Interestingly, fever is often absent when patients first seek medical care for this strain, even though about half report having had a fever earlier. If you develop bloody diarrhea with severe cramping, especially after eating undercooked meat or visiting a setting where outbreaks occur, getting a stool culture is important, because standard lab tests don’t always detect every pathogen.
Upper Digestive Tract Bleeding
When bleeding starts in the stomach or esophagus, it produces black, tarry stools rather than red blood. Stomach ulcers are the leading cause. These form when the stomach’s protective lining breaks down, often from long-term use of anti-inflammatory painkillers or from a bacterial infection called H. pylori.
Inflammation of the esophagus (from chronic acid reflux, for example) is another common source. Less commonly, people with advanced liver disease develop enlarged blood vessels in the esophagus or stomach called varices, which can bleed heavily. A Mallory-Weiss tear, a small rip at the junction of the stomach and esophagus caused by forceful vomiting or retching, can also cause upper tract bleeding. One note: certain foods and supplements can mimic the appearance of upper tract bleeding. Iron supplements, bismuth (the active ingredient in Pepto-Bismol), and even large amounts of blueberries can turn stool very dark. If you recently started any of these, that may be the explanation.
Colorectal Cancer and Polyps
This is usually the fear behind the search, so it’s worth putting into perspective. Colorectal cancer can cause blood in the stool, but it is a far less common cause than hemorrhoids, fissures, or diverticular disease. Cancer-related bleeding may be bright red (if the tumor is in the rectum or lower colon) or darker (if higher up). It can also be invisible to the naked eye, showing up only on screening tests.
Other warning signs that raise the index of suspicion include a persistent change in bowel habits (new constipation or diarrhea lasting more than a few weeks), unexplained weight loss, a feeling that your bowel doesn’t empty completely, and progressive fatigue from slow, chronic blood loss. Polyps, which are precancerous growths on the colon lining, can also bleed and are one reason routine screening matters.
The U.S. Preventive Services Task Force recommends colorectal cancer screening starting at age 45 for average-risk adults. The fecal immunochemical test, a simple at-home stool test, has a sensitivity of about 96 percent for detecting colorectal cancer, meaning it catches nearly all true cases. A negative result is reassuring, with a negative predictive value of 99.8 percent. For anyone experiencing new or unexplained rectal bleeding, especially over age 45 or with a family history of colorectal cancer, a colonoscopy provides the most definitive answer.
When Rectal Bleeding Is an Emergency
Most episodes of blood in the stool are not emergencies, but some are. Seek emergency care if rectal bleeding is continuous, heavy, or accompanied by severe abdominal pain or cramping. The Mayo Clinic identifies several signs of shock that warrant calling 911: rapid or shallow breathing, dizziness or lightheadedness when standing, blurred vision, fainting, confusion, nausea, cold or clammy skin, and very low urine output. These symptoms suggest significant blood loss and require immediate treatment.
For smaller amounts of blood that recur over days or weeks, or for a single episode that resolves on its own, scheduling an appointment with your doctor is reasonable. The key is not to dismiss it. Even when the most likely cause is something benign, confirming that with a proper evaluation gives you a clear answer and, when needed, a path to treatment.