Blood in your stool usually comes from somewhere along your digestive tract, and the color tells you a lot about where. Bright red blood typically originates from the lower digestive tract (colon, rectum, or anus), while black, tarry stools point to bleeding higher up, like the stomach or small intestine. The most common cause by far is hemorrhoids, but several other conditions can be responsible.
What the Color Tells You
Bright red blood on the toilet paper, in the bowl, or coating the stool means the bleeding source is relatively close to the exit. The blood hasn’t had time to be broken down, so it keeps its color. This is the type most people notice and worry about.
Black, tarry, sticky stools are a different signal. When blood starts higher in the digestive tract, your stomach acid and digestive enzymes break down hemoglobin (the protein that makes blood red) as it travels through your intestines. By the time it reaches the other end, it’s turned dark and tar-like. This type of bleeding is harder to recognize because it doesn’t look like blood at first glance.
Before you panic, though, consider what you’ve eaten or taken recently. Beets and foods with red coloring can make stools look reddish. Black licorice, blueberries, iron supplements, activated charcoal, and bismuth-based medications like Pepto-Bismol can all turn stool black. If any of these apply, that may be your entire explanation.
Hemorrhoids and Anal Fissures
These two conditions account for the vast majority of bright red rectal bleeding, especially in younger adults. Internal hemorrhoids develop inside the lower rectum. They don’t usually hurt, but they bleed, often enough to turn the toilet water red or leave blood on the paper. You might not even know you have them until you see the blood.
Anal fissures are small tears in the lining of the anus, usually caused by passing hard or large stools. They produce bright red blood too, but unlike hemorrhoids, fissures tend to cause a sharp, stinging pain during and after a bowel movement. Both conditions are common, treatable, and not dangerous on their own. Increasing fiber and water intake, and avoiding straining, resolves most cases over a few weeks.
Infections That Cause Bloody Diarrhea
If the blood showed up alongside diarrhea, fever, or cramping that came on suddenly, an intestinal infection is a likely culprit. Several bacteria are known to cause bloody stool: Salmonella, Campylobacter, Shigella, and a strain of E. coli called O157:H7 (often linked to undercooked ground beef or contaminated produce). Bloody stool is especially common with Shiga toxin-producing E. coli infections.
The pattern that suggests an infection rather than something structural is the combination of fever, an urgent need to go, and visibly bloody loose stools. Most bacterial gut infections resolve on their own within a week, though some require treatment. If you’ve recently traveled, eaten something questionable, or been on antibiotics (which can trigger a C. difficile infection), those details help narrow things down.
Inflammatory Bowel Disease
Crohn’s disease and ulcerative colitis cause chronic inflammation in the digestive tract. The hallmark pattern is bloody diarrhea that keeps coming back, paired with abdominal pain and unintentional weight loss. Unlike a one-time infection, IBD tends to flare and remit over months or years. If you’ve been dealing with recurring episodes of bloody stool, especially combined with fatigue or joint pain, this is worth investigating with a doctor.
Diverticular Bleeding
Diverticulosis, where small pouches form in the colon wall, is extremely common in people over 50. Most of the time these pouches cause no trouble. Occasionally, though, a blood vessel near one of them breaks open and produces a sudden, heavy bleed. The distinctive feature of diverticular bleeding is that it’s painless. You pass a large amount of bright or dark red blood without any cramping. It often stops on its own but can be alarming because of the volume.
Stomach Ulcers and Upper GI Bleeding
Peptic ulcers form when stomach acid erodes the lining of your stomach or the upper part of your small intestine. When an ulcer bleeds, the blood travels the full length of your digestive tract, arriving as black, tarry stool. You might also have a dull or burning stomach pain (often between meals or at night), nausea, bloating, or heartburn.
Certain medications raise your risk significantly. NSAIDs like ibuprofen and naproxen roughly 2.5 times the risk of gastrointestinal bleeding. Blood thinners (anticoagulants) carry about 4 times the risk. If you take any of these regularly and notice dark stools, that combination deserves prompt attention. Low-dose aspirin, interestingly, does not appear to carry the same level of risk for mid-gut bleeding.
Colorectal Cancer
This is the possibility most people are worried about when they search this question. Colorectal cancer can cause blood in the stool, but it’s far less common than hemorrhoids or fissures, particularly in people under 45. The bleeding from a tumor is often slow and invisible to the naked eye, though it can sometimes produce visible red or dark blood mixed into the stool.
Routine screening is now recommended starting at age 45 for average-risk adults, per the U.S. Preventive Services Task Force. Between ages 50 and 75, screening is strongly recommended. The most accessible screening tool is a fecal immunochemical test (FIT), a home stool test that detects hidden blood. A single FIT test catches about 80% of colorectal cancers, a meaningful improvement over older stool tests. A colonoscopy remains the most thorough option and is typically done every 10 years.
Signs that push rectal bleeding beyond “probably hemorrhoids” and toward further evaluation include a change in bowel habits lasting more than a few weeks, unexplained weight loss, a feeling that your bowel doesn’t fully empty, stools that become persistently thinner, or bleeding that doesn’t match the typical hemorrhoid pattern.
When Bloody Stool Is an Emergency
Most rectal bleeding is not an emergency, but heavy or continuous bleeding can become one. If you’re passing large amounts of blood and experience any of the following, call 911 or get to an emergency room:
- Dizziness or lightheadedness when you stand up
- Rapid, shallow breathing
- Cold, clammy, or pale skin
- Confusion or fainting
- Blurred vision
- Very low urine output
These are signs of significant blood loss. Bleeding accompanied by severe abdominal pain or cramping also warrants an immediate ER visit, even if you feel otherwise stable. Continuous or heavy bleeding that isn’t slowing down on its own needs evaluation regardless of other symptoms.