Blood in your stool usually comes from somewhere along the digestive tract, and the color tells you a lot about where the bleeding started. Most cases trace back to common, treatable problems like hemorrhoids or small tears near the anus. But because the same symptom can occasionally signal something more serious, understanding the possible causes helps you figure out what to do next.
What the Color of the Blood Means
The shade of blood you see is a rough map of where the bleeding originates. Bright red blood on toilet paper or coating the stool typically comes from the lower end of the digestive tract: the rectum, anus, or the last portion of the colon. This is the most common presentation and often the least worrying.
Dark, maroon-colored blood mixed into the stool usually points to bleeding higher up in the colon. If the blood has been sitting in the intestines long enough to be partially digested, it turns the stool black and tarry with a distinctly foul smell. Black tarry stools generally indicate bleeding from the stomach or upper small intestine, where stomach acid breaks down the blood before it moves through.
There are exceptions. Very rapid bleeding from a stomach ulcer can move through the gut so fast that it comes out bright red. And slow-moving blood from the right side of the colon can appear dark or tarry even though the source is technically in the lower tract. Color is a useful clue, not a definitive diagnosis.
The Most Common Causes
Hemorrhoids and Anal Fissures
Hemorrhoids are swollen blood vessels in and around the rectum. They’re extremely common, especially after age 30, during pregnancy, and in people who strain during bowel movements. The bleeding is typically bright red, painless, and noticed on the toilet paper or dripping into the bowl. Anal fissures, which are small tears in the lining of the anus, produce similar bright red blood but usually come with a sharp, stinging pain during and after a bowel movement. Both conditions improve with increased fiber intake, hydration, and avoiding prolonged straining.
Diverticular Bleeding
Diverticulosis is a condition where small pouches form in the wall of the colon, typically in people over 50. These pouches develop at weak spots where blood vessels penetrate the colon wall. Over time, the tissue covering those vessels can erode and rupture, causing sudden, painless bleeding that can be surprisingly heavy. The blood is often maroon or bright red. Most diverticular bleeds stop on their own, but they can recur.
Inflammatory Bowel Disease
In ulcerative colitis and Crohn’s disease, the immune system becomes overactive and sends white blood cells into the lining of the digestive tract. These cells release inflammatory chemicals that damage tissue, causing pain, diarrhea, and bleeding. Ulcerative colitis affects the colon and rectum continuously, so bloody diarrhea is one of its hallmark symptoms. Crohn’s disease can affect any part of the digestive tract and may or may not involve visible blood, depending on where the inflammation sits. Both conditions tend to flare and remit over time, and blood in the stool during a flare is common.
Infections
Certain bacterial infections cause bloody diarrhea, usually after eating contaminated food or water. E. coli (the hemorrhagic type) typically starts as painful, watery diarrhea that becomes bloody within one to three days, with symptoms appearing about three days after exposure. Shigella infections have a shorter incubation period of one to three days and produce blood and mucus in the stool along with an intense, painful urge to go. Salmonella occasionally causes bloody stools too, though it’s less common, with symptoms starting 8 to 72 hours after exposure. These infections usually resolve within a week, but the bloody diarrhea can be alarming while it lasts.
Colorectal Polyps and Cancer
Polyps are growths on the inner lining of the colon that can slowly bleed as they enlarge. Most polyps are benign, but some develop into colorectal cancer over years. Cancer-related bleeding is often intermittent and can be so subtle that you don’t see it with the naked eye. Other signs that point toward something more concerning include unexplained weight loss, a persistent change in bowel habits, narrower stools, or a feeling that the bowel doesn’t empty completely. The U.S. Preventive Services Task Force recommends that adults begin screening at age 45 and continue through age 75.
Medications That Increase Bleeding Risk
Common pain relievers can damage the digestive tract and cause bleeding you might not expect. NSAIDs like ibuprofen, naproxen, and aspirin work by blocking an enzyme that, among other things, helps maintain a protective mucus layer in the stomach and intestines. Without that protection, the lining becomes vulnerable to stomach acid, leading to erosions, ulcers, and bleeding.
Not all NSAIDs carry equal risk. A large meta-analysis found that ibuprofen roughly doubled the risk of GI bleeding compared to not taking any NSAID. Naproxen carried about four times the risk. Indomethacin and piroxicam were associated with even higher odds, at roughly five and nine times the baseline risk, respectively. Blood thinners and anticoagulants compound the problem further if you’re taking them alongside an NSAID. If you notice dark or bloody stools while using these medications regularly, that’s worth bringing up promptly.
How Doctors Find the Source
When the cause isn’t obvious from your symptoms, doctors use a few tools to pinpoint where the bleeding is coming from. A colonoscopy, where a flexible camera examines the entire colon, is the gold standard. It detects advanced growths and cancers with about 98.5% sensitivity and can treat some problems on the spot by removing polyps or cauterizing bleeding vessels.
For screening purposes, a fecal immunochemical test (FIT) detects hidden blood in a stool sample you collect at home. It’s convenient and highly specific, meaning a positive result is a reliable signal that something needs investigation. However, its sensitivity for catching advanced growths is much lower, around 16%, so it works best as a repeated annual screen rather than a one-time diagnostic. A positive FIT is always followed up with a colonoscopy.
For upper GI bleeding (suggested by black tarry stools), an upper endoscopy examines the esophagus, stomach, and upper small intestine. In cases of rapid or hard-to-locate bleeding, imaging scans or specialized procedures can identify the source.
Signs That Need Immediate Attention
A small amount of bright red blood on the toilet paper after straining is common and rarely an emergency. But heavy bleeding, or bleeding paired with other symptoms, changes the picture. Feeling lightheaded, dizzy, or faint alongside bloody stool suggests enough blood loss to affect circulation. Shortness of breath is another warning sign.
Severe GI bleeding can progress to shock. The signs include a fast heart rate, pale skin, cold hands and feet, sweating, confusion, or loss of consciousness. Black tarry stools that persist over multiple bowel movements also warrant urgent evaluation, since they often indicate active bleeding from the stomach or upper intestine that may not stop on its own.
Blood in the stool that keeps coming back over days or weeks, even in small amounts, is worth investigating regardless of how minor it looks. Intermittent bleeding is exactly how many significant conditions, including early-stage colorectal cancer, first show up.