Blood in your stool most commonly comes from hemorrhoids, which are swollen veins in your rectum or anus. But the color, amount, and pattern of bleeding can point to a range of causes, from a simple tear in the skin to conditions that need medical attention. Understanding what different types of bleeding look like and what triggers them can help you figure out what’s going on.
What the Color of the Blood Tells You
The color of blood in your stool is one of the most useful clues about where the bleeding is coming from. Bright red blood typically originates lower in the digestive tract, in the colon, rectum, or anus. Dark, black, or tarry stools usually mean the bleeding started much higher up, in the stomach or upper small intestine. Blood from higher up has more time to be broken down by digestive acids as it travels through, which turns it dark.
Maroon-colored blood often falls somewhere in between, suggesting a source in the middle of the digestive tract. Knowing this distinction matters because the possible causes differ significantly depending on where the bleeding originates.
The Most Common Causes
Hemorrhoids are the single most common reason for blood in the stool. They develop when veins in the rectum or anus swell, usually from straining during bowel movements due to constipation. You might notice bright red blood on the toilet paper, on the surface of your stool, or dripping into the bowl. Hemorrhoids are rarely dangerous, though they can be uncomfortable and persistent.
Anal fissures are another frequent culprit. These are small tears in the lining of the anal canal, also often caused by straining or passing hard stools. They tend to cause sharp pain during a bowel movement along with a streak of bright red blood. Most fissures heal on their own within a few weeks with softer stools and proper hygiene.
Diverticulitis occurs when small pouches that form in the colon wall become infected and inflamed. The inflammation makes blood vessels inside those pouches fragile, and they can rupture. Diverticular bleeding can be sudden and sometimes heavy, producing a noticeable amount of bright red or maroon blood. This is more common in older adults.
Upper Digestive Tract Bleeding
Peptic ulcers are open sores in the stomach or upper small intestine that form when stomach acid erodes the protective lining. When an ulcer bleeds, the blood is digested as it passes through the intestines, producing stools that look dark, black, and tarry. This type of stool has a distinct, unusually foul smell that’s different from normal bowel movements.
Pain medications like ibuprofen, naproxen, and aspirin can contribute to this kind of bleeding. These drugs work by blocking inflammation, but in doing so they also weaken the protective barrier of the stomach and intestinal lining. They can also reduce the blood’s ability to clot. People who take both a pain reliever like ibuprofen and low-dose aspirin together have roughly four to five times the risk of lower digestive tract bleeding compared to people taking neither. Even over-the-counter doses taken regularly can cause problems over time.
Blood thinners prescribed for heart conditions or blood clots don’t cause ulcers themselves, but they can make any existing source of bleeding worse and harder to stop.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are two forms of inflammatory bowel disease that can cause blood in the stool, though they behave differently. Ulcerative colitis affects the large intestine and commonly causes bloody diarrhea, often mixed with mucus. It tends to flare and remit in cycles.
Crohn’s disease primarily affects the small intestine, and its diarrhea is usually not bloody. However, Crohn’s can develop anywhere in the digestive tract, and when it involves the colon, bleeding becomes more likely. Both conditions typically come with other symptoms: cramping, urgency, fatigue, and weight loss over weeks or months rather than a single episode of bleeding.
Polyps and Colorectal Cancer
Colon polyps are small growths on the inner lining of the colon. Most are harmless, but some can become cancerous over time. Both polyps and colorectal cancer can bleed when stool rubs against them as it passes through. This bleeding is often slow and invisible to the naked eye, meaning your stool might look completely normal even though it contains trace amounts of blood. That’s why screening tests that detect hidden blood are used as a first-line tool for colorectal cancer detection.
The risk of colorectal cancer in someone with rectal bleeding varies significantly by age. In a large study of primary care patients aged 30 to 99, cancer risk among those with rectal bleeding was relatively low in younger adults but climbed steadily with age. Among men around age 80 who presented with rectal bleeding, the risk of colon cancer was approximately 1.7%. For women in the same age group, it was around 1.3%. These numbers mean the vast majority of people with rectal bleeding do not have cancer, but the risk is real enough, especially over age 50, that it shouldn’t be ignored.
Abnormal Blood Vessels in the Gut
In older adults, a condition called angiodysplasia can cause unexplained bleeding. Over time, the small blood vessels in the intestinal wall can become stretched and twisted due to repeated pressure changes. These fragile, dilated vessels are prone to leaking blood into the digestive tract. The bleeding is often intermittent, which can make it tricky to diagnose. People with certain heart valve problems are at higher risk because the condition disrupts proteins involved in blood clotting.
Foods and Supplements That Mimic Blood
Before assuming the worst, consider what you’ve eaten recently. Several common foods and supplements can make stool look bloody when it isn’t. Beets contain a red pigment called betanin that can turn stool a startling blood-red color. Blueberries in large quantities can make stool so dark it appears almost black. Black licorice and heavily dyed candy can do the same.
On the supplement side, iron tablets commonly turn stool dark green or black, which can easily be mistaken for the tarry appearance of digested blood. Bismuth subsalicylate, the active ingredient in Pepto-Bismol, turns stool jet black. If you’ve taken any of these in the past day or two and your stool looks alarming but you feel fine otherwise, that’s likely the explanation.
Signs That Bleeding Is Serious
Most causes of blood in the stool are not emergencies. A small amount of bright red blood after straining, especially if you’ve been constipated, is very commonly from hemorrhoids or a fissure. But certain patterns warrant urgent attention.
Heavy bleeding that doesn’t stop, large clots, or stool that is entirely dark and tarry suggests significant blood loss from somewhere in the digestive tract. If heavy bleeding is accompanied by a fast heart rate, pale skin, cold hands, sweating, confusion, or feeling faint, these are signs of shock from blood loss. This is a medical emergency.
Even without dramatic symptoms, bleeding that recurs over several weeks, happens alongside unexplained weight loss, or is accompanied by a persistent change in bowel habits (new constipation, thinner stools, ongoing diarrhea) deserves evaluation. These patterns are the ones most associated with conditions like polyps, inflammatory bowel disease, or cancer, all of which are more treatable when caught early.