Blood in your stool can look very different depending on where in the digestive tract it’s coming from. It might be bright red on the toilet paper, dark red mixed into the stool, or cause your stool to turn black and tarry. Sometimes blood is invisible to the naked eye entirely. Knowing what to look for helps you figure out what’s going on and how urgently you need to act.
What Blood in Stool Actually Looks Like
The color and location of the blood tells you a lot. Bright red blood that coats the outside of the stool, drips into the toilet bowl, or shows up on toilet paper usually points to a source near the end of the digestive tract, like hemorrhoids or a tear in the anal lining. Blood that’s mixed into the stool itself suggests the source is higher up, typically somewhere in the colon.
Black, tarry stools are a different signal altogether. This happens when blood has been digested as it travels through the stomach and small intestine. It takes roughly 50 milliliters of blood in the stomach to turn stools black. Tarry stools have a distinctive stickiness and a strong, unpleasant smell that most people notice right away. This appearance usually means the bleeding is coming from the upper digestive tract: the stomach, esophagus, or upper small intestine.
There’s one important exception. Very rapid bleeding from the upper digestive tract, such as from a stomach ulcer, can move through so quickly that the blood comes out bright red. So red blood doesn’t always mean a lower source.
Foods and Medications That Mimic Blood
Before jumping to conclusions, consider what you’ve eaten or taken recently. Beets and foods with red coloring can make stool look reddish. On the darker end, iron supplements, black licorice, blueberries, activated charcoal, and bismuth-containing medications (like Pepto-Bismol) can all turn stool black. These color changes are harmless, but they can look alarming. A simple stool test at your doctor’s office can confirm whether actual blood is present.
Blood You Can’t See
Not all blood in the stool is visible. Small amounts of bleeding, often called occult (hidden) blood, can go on for weeks or months without any obvious change in stool color. The only clues may be indirect: unexplained fatigue, feeling unusually short of breath during normal activities, or looking pale. These are signs of anemia from slow, steady blood loss. A fecal occult blood test is the only way to catch this type of bleeding.
Two types of at-home stool tests exist. The older guaiac-based test (FOBT) detects blood through a chemical reaction, while the newer fecal immunochemical test (FIT) specifically detects human hemoglobin. FIT generally performs better, with fewer false positives and false negatives. FIT also doesn’t require dietary restrictions before the test, which makes it more practical. The U.S. Preventive Services Task Force recommends routine colorectal cancer screening starting at age 45 for people at average risk, continuing through age 75. Stool-based tests like FIT are one of the accepted screening methods.
Common Causes by Location
Near the Anus
Hemorrhoids are the single most common cause of rectal bleeding, especially in middle-aged and older adults. Internal hemorrhoids typically cause painless bleeding: you’ll see bright red blood on the toilet paper or in the bowl, but won’t feel much discomfort. Anal fissures, which are small tears in the lining of the anal canal, tend to happen with constipation or the passage of hard stools. These cause painful bowel movements and leave blood streaked on the surface of the stool.
In the Colon
Colon polyps, which are small growths on the inner wall of the colon, can bleed intermittently. Some polyps are precancerous, which is why screening matters. Diverticular disease involves small pouches that form in the colon wall. The blood vessels in these pouches become fragile over time and can rupture, sometimes causing sudden, painless, and surprisingly heavy bleeding. Inflammatory bowel diseases like ulcerative colitis and Crohn’s disease cause chronic inflammation that leads to bleeding, often alongside diarrhea and abdominal pain.
In the Upper Digestive Tract
Stomach ulcers, esophageal problems, and other upper GI conditions produce the black, tarry stools described earlier. If you’re vomiting material that looks like coffee grounds alongside dark stools, the source is almost certainly in the stomach or esophagus.
Colorectal cancer accounts for roughly 3.4% of rectal bleeding cases. It’s not the most likely cause, but it’s the most important one to rule out, particularly if you’re over 45 or have a family history.
Signs That Bleeding Is Serious
Some symptoms alongside bloody stool signal significant blood loss and need immediate attention. Lightheadedness or dizziness when standing, a racing heart rate, fainting, and noticeable weakness all suggest your body is losing blood faster than it can compensate. Passing large amounts of bright red blood or clots, or noticing persistent black tarry stools over multiple bowel movements, also warrants urgent evaluation.
The combination of abdominal pain with bloody stool can point to conditions like ischemic colitis (reduced blood flow to the colon), infections, or inflammatory bowel disease, all of which benefit from prompt diagnosis.
What Happens During Evaluation
If you report blood in your stool, the first step is usually a physical exam that includes a rectal examination. This alone can identify hemorrhoids, fissures, or masses near the opening of the rectum. Your doctor will likely order blood work to check for anemia and get a sense of how much blood you may have lost.
A colonoscopy is the most thorough way to examine the entire colon and identify the source of bleeding. During the procedure, a flexible camera is passed through the colon, and if polyps or other abnormalities are found, they can often be removed or biopsied on the spot. For bleeding that appears to come from the upper digestive tract, an upper endoscopy examines the esophagus, stomach, and upper small intestine using a similar approach from the other end.
Not every instance of blood in the stool requires a colonoscopy. A younger person with obvious hemorrhoid symptoms and a small amount of bright red blood on the toilet paper may only need a targeted exam. But persistent bleeding, a change in bowel habits, unexplained weight loss, or age over 45 without recent screening all push toward more complete evaluation.