Blood in your stool can look very different depending on where it’s coming from. Sometimes it’s obvious: bright red streaks on toilet paper or in the bowl. Other times, it shows up as a black, sticky stool that looks nothing like blood at all. And in some cases, the amount is so small you can’t see it without a lab test. Here’s how to tell what you’re dealing with and what it means.
What Blood in Stool Actually Looks Like
The color and texture of blood in your stool tells you a lot about where the bleeding is happening. There are two main types you might notice.
Bright red blood, whether on the toilet paper, dripping into the bowl, or mixed into the stool, typically comes from the lower part of your digestive tract: your colon, rectum, or anus. This is the type most people recognize immediately.
Black, tarry stools are the less obvious sign. When blood originates higher up, in your stomach or the top of your small intestine, it gets digested on the way down and turns dark. The result is a stool that’s not just dark but distinctly sticky, with a strong, unpleasant smell. It takes roughly 50 ml of blood in the stomach (about 3 tablespoons) to turn stools black. If you’ve ever seen it, you likely remember the texture.
In rare cases, a very fast bleed from the upper digestive tract can still come out bright red, because the blood moves through too quickly to be digested. This tends to happen with arterial bleeding from something like a peptic ulcer and usually involves a large volume.
Foods and Medications That Mimic Blood
Before you panic, consider what you’ve eaten or taken recently. Beets and foods with red food coloring can make stool look reddish, closely mimicking the appearance of blood. On the dark side, iron supplements, activated charcoal, black licorice, blueberries, and bismuth-based medications like Pepto-Bismol can all turn your stool black. The key difference: stools darkened by these substances generally aren’t sticky or tarry the way true blood-containing stools are, and they don’t have the same foul odor.
Blood You Can’t See
Not all bleeding is visible. Small amounts of blood, called occult blood, can be present in your stool without any color change at all. You won’t notice it on the toilet paper or in the bowl. The only clue might be indirect: unexplained fatigue, shortness of breath with mild exertion, or pale skin, all signs of iron-deficiency anemia caused by slow, chronic blood loss.
Occult blood is detected through a lab test called a fecal occult blood test (FOBT) or a fecal immunochemical test (FIT). The FIT version is more commonly used now because it’s more accurate, with a specificity around 92% for detecting serious growths compared to about 76% for the older guaiac-based test. These are the same tests used in routine colorectal cancer screening.
If you’re collecting a sample at home, store it at room temperature and get it to the lab within 24 hours. The process involves catching the sample in a collection device placed over the toilet bowl, transferring a small amount to the test container, and washing your hands thoroughly before and after.
Most Common Causes of Visible Blood
Hemorrhoids are the single most common reason people see blood in their stool. These are swollen veins inside or around the anus, and the bleeding is typically bright red, painless, and noticed on the toilet paper or dripping into the bowl. You might also feel a lump near the anus or notice itching.
Anal fissures, small tears in the lining of the anal canal, are the other frequent culprit. The difference from hemorrhoids is pain. Fissures tend to cause a sharp, stinging pain during bowel movements that can linger afterward. The blood is also bright red, but the discomfort is usually the more noticeable symptom.
Beyond these two, other causes include:
- Diverticulitis: infected or inflamed pockets in the colon wall, usually in the lower section, which can cause sudden, painless, sometimes heavy bleeding
- Inflammatory bowel disease (IBD): chronic conditions like Crohn’s disease or ulcerative colitis that cause ongoing inflammation, diarrhea, and bleeding
- Colon polyps and colorectal cancer: growths on the colon lining that can bleed, especially larger ones or those that have become cancerous
- Peptic ulcers: sores in the stomach or upper small intestine, more likely to cause black, tarry stools
- Infections: bacterial infections like E. coli or C. diff can cause bleeding from the colon
What a Positive Test Actually Means
If a screening test finds hidden blood in your stool, the next step is usually a colonoscopy to look directly at the lining of your colon. But a positive result doesn’t automatically mean something serious. In screening populations, about 40% of people with a positive fecal blood test are found to have a polyp or colorectal cancer on colonoscopy. That means the majority have a different explanation, or no clear source is found at all.
Among those who do have a finding, polyps (adenomas) are far more common than cancer. In one large Italian study of over 5,500 people with positive screening tests, about 23% had adenomas and roughly 6% had colorectal cancer. Polyps are typically removed during the colonoscopy itself, which is one reason screening is so effective at prevention.
How Doctors Evaluate Rectal Bleeding
When you report blood in your stool, the evaluation typically starts with your history: what the blood looked like, whether you have pain, any changes in bowel habits, unexplained weight loss, fatigue, or a family history of colorectal cancer. These details help determine how urgently you need further testing.
Rectal bleeding on its own has a positive predictive value for colorectal cancer of only about 8% in people over 50 presenting to a primary care doctor. That number is reassuringly low, but it’s high enough that investigation is warranted, especially in older adults or anyone with additional red-flag symptoms. For younger, lower-risk patients with bleeding that looks consistent with hemorrhoids or fissures, a limited exam of the lower colon may be sufficient. Older patients or those with weight loss, changing bowel habits, or family history typically need a full colonoscopy.
One important note: rectal bleeding is never caused by irritable bowel syndrome. If you’ve been told you have IBS and you start seeing blood, that’s a separate issue that needs its own evaluation.
Signs That Bleeding Needs Emergency Care
Most rectal bleeding is not an emergency, but heavy or continuous bleeding with certain other symptoms signals a serious situation. Call emergency services if you notice rectal bleeding along with any of these: 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 are signs your body is losing enough blood to affect circulation. Continuous or heavy bleeding paired with severe abdominal pain or cramping also warrants an immediate trip to the emergency room, even without those other symptoms.