Blood in your stool usually comes from somewhere along the digestive tract, and the cause ranges from something minor like a small tear in the skin around the anus to something that needs prompt medical attention like an ulcer or inflammatory bowel disease. In most people under 30, the source turns out to be a hemorrhoid or anal fissure. But because the list of possible causes is long, the color, amount, and pattern of bleeding all matter when figuring out what’s going on.
What the Color of the Blood Tells You
Bright red blood, especially on toilet paper or coating the outside of the stool, typically comes from the lower end of the digestive tract: the rectum or anus. The closer the bleeding source is to the exit, the redder the blood looks because it hasn’t had time to be broken down by digestive enzymes.
Dark red or maroon-colored blood mixed into the stool usually points to a source higher up in the colon. Black, tarry stool with a distinct foul smell suggests bleeding from the stomach or upper small intestine. Blood that travels that far through the digestive system gets chemically altered along the way, turning it dark. Vigorous upper GI bleeding can occasionally produce bright red blood if it moves through the intestines fast enough, but that’s uncommon and usually involves a large volume of blood loss.
Hemorrhoids and Anal Fissures
These two conditions are the most common reasons people see blood after a bowel movement, particularly in younger adults. They account for the majority of anorectal bleeding, and both are tied to straining, hard stools, or chronic constipation.
Hemorrhoids are swollen veins around the anus. They develop when too much pressure builds up in that area, whether from straining during bowel movements, sitting for long periods, pregnancy, or heavy lifting. Most hemorrhoids don’t actually hurt. You might notice bright red blood on the toilet paper or dripping into the bowl, sometimes with mild discomfort or itching, but no sharp pain.
Anal fissures, by contrast, are small tears in the skin lining the anus. They tend to cause a sharp, burning pain during and after a bowel movement. You’ll see blood when you poop or wipe, and the area may itch or burn between bowel movements. Fissures often heal on their own within a few weeks if stools stay soft, but they can become chronic if the tear keeps reopening.
Diverticular Bleeding
Diverticular disease becomes increasingly common after age 40. Small pouches called diverticula form along the colon wall, usually without causing any symptoms. Bleeding happens when a small artery running alongside one of these pouches gets eroded, releasing blood directly into the colon. The result can be dramatic: a sudden, painless gush of dark red or maroon blood. It often stops on its own, but the volume can be alarming enough to send people to the emergency room. Diverticular bleeding is one of the most common causes of significant lower GI bleeding in older adults.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both involve chronic inflammation that damages the intestinal lining and can cause bleeding. In ulcerative colitis, inflammation typically starts in the rectum and spreads continuously through the colon. Bloody diarrhea, often with mucus, is a hallmark symptom.
Crohn’s disease works differently. It causes inflammation in patches that can affect any part of the digestive tract and all layers of the intestinal wall. Over time, the inflammation thickens the intestinal walls and causes deep ulcers that can crack outward, forming tunnels or passageways through the tissue. Bleeding from Crohn’s may be less visible than in ulcerative colitis, but it can still cause blood in the stool alongside abdominal pain, diarrhea, fatigue, and weight loss. Both conditions are chronic and require ongoing management.
Stomach and Duodenal Ulcers
Ulcers in the stomach and upper small intestine together account for up to 50% of upper GI bleeding. When these ulcers erode into a blood vessel, the blood passes through the rest of the digestive system and typically comes out as black, tarry stool rather than red blood. You might also experience burning stomach pain, nausea, or vomiting. Upper GI bleeds from ulcers can range from slow and chronic (causing anemia over time) to sudden and severe.
Colorectal Polyps and Cancer
Polyps are growths on the inner lining of the colon that can bleed intermittently, often producing small amounts of blood that you may not even notice. Some polyps eventually become cancerous if left in place, which is why screening colonoscopies are recommended starting at age 45.
Colorectal cancer can cause persistent changes in bowel habits, unexplained weight loss, fatigue from blood loss, and visible or hidden blood in the stool. The risk increases substantially with age. About 26% of new cases occur in people aged 65 to 74, and another 22% in people aged 55 to 64, according to National Cancer Institute data. Only about 2% of cases occur in people under 35. That said, rates in younger adults have been rising in recent years, so blood in the stool at any age deserves attention, especially if it persists or comes with other symptoms like unintended weight loss or a change in stool caliber.
Medications That Cause GI Bleeding
Common over-the-counter pain relievers are a frequently overlooked cause. NSAIDs like ibuprofen, naproxen, and diclofenac can directly damage the lining of the stomach and intestines. These drugs are weak acids that penetrate the protective mucus layer, disrupt the surface cells, and trigger erosions that bleed. Even low-dose aspirin carries meaningful risk. One large analysis found that aspirin was responsible for 18% of hospital admissions for adverse drug reactions, and GI bleeding accounted for 72% of those aspirin-related admissions.
The risk is highest in people who take NSAIDs regularly, use more than one at a time, or combine them with blood thinners. In the large intestine, these drugs can also cause inflammation, increased gut permeability, and bleeding. If you take NSAIDs frequently and notice dark or bloody stool, that connection is worth investigating.
Foods That Mimic Blood in Stool
Not every alarming-looking stool contains actual blood. Beets and foods with red coloring can make stool appear reddish. On the other end, black licorice, blueberries, iron supplements, activated charcoal, and bismuth-containing medications like Pepto-Bismol can all turn stool black and mimic the appearance of upper GI bleeding. A simple chemical test at a doctor’s office can confirm whether blood is actually present, which is worth doing if you’re unsure.
How Doctors Find the Source
The approach depends on how fast you’re bleeding and where the blood seems to be coming from. For most people with small amounts of bright red blood and an otherwise low-risk profile, the evaluation may start with a physical exam of the anorectal area and a look at the lower rectum with a small scope.
Colonoscopy is the primary tool for evaluating lower GI bleeding and identifies a definitive source in more than 70% of patients. If there’s reason to suspect the bleeding originates in the stomach or upper intestine, an upper endoscopy is performed first. In cases of massive rectal bleeding, doctors will often check for an upper GI source early on, since heavy bleeding from a stomach ulcer can sometimes pass through quickly enough to come out looking like lower GI bleeding.
Signs That Bleeding Is an Emergency
A small streak of blood on toilet paper after straining is a very different situation from passing large amounts of blood or clots. Seek emergency help if rectal bleeding is heavy or continuous, or if it comes with any of these signs of significant blood loss: rapid or shallow breathing, dizziness or lightheadedness when you stand up, blurred vision, fainting, confusion, nausea, cold or clammy skin, or very low urine output. These symptoms suggest the body is losing blood faster than it can compensate, and that requires immediate evaluation.
Even without those alarm signs, blood in the stool that recurs over days or weeks, appears without an obvious explanation, or is accompanied by weight loss, persistent abdominal pain, or a change in bowel habits warrants a medical workup. The cause is often something treatable, but identifying it matters.