Blood in your stool is usually caused by something minor like hemorrhoids or a small tear in the skin around your anus. But it can also signal infections, inflammatory conditions, or in some cases, something more serious like colorectal cancer. The color of the blood, how much there is, and whether you have other symptoms all help narrow down what’s going on.
What the Color of the Blood Tells You
Bright red blood typically comes from the lower part of your digestive tract: the rectum, anus, or the last stretch of your colon. You might see it on the toilet paper, dripping into the bowl, or coating the surface of your stool. This is the most common presentation, and in many cases it points to a benign cause close to the exit.
Dark, tarry, almost black stool is a different situation. That color means blood has been partially digested as it traveled through your system, which usually places the source higher up, in the stomach or small intestine. Tarry stool tends to have a distinctly foul smell. It can occasionally come from the right side of the colon as well, but it always warrants prompt medical evaluation because upper GI bleeding can be significant.
Maroon-colored stool falls somewhere in between and often points to bleeding in the middle of the digestive tract or a fast-moving bleed from higher up.
Hemorrhoids and Anal Fissures
These two causes account for the majority of bright red rectal bleeding, especially in otherwise healthy adults. Hemorrhoids are swollen blood vessels inside or just outside the anus. Most hemorrhoids don’t cause pain, which is why seeing blood can feel alarming when nothing else seems wrong. You might notice blood on the tissue or in the bowl after a bowel movement, along with mild discomfort or itching.
Anal fissures are small tears in the lining of the anus, usually caused by passing a hard or large stool. Unlike hemorrhoids, fissures tend to hurt, sometimes sharply, during and after you poop. The blood looks similar (bright red, on the paper or stool surface), but the pain during bowel movements is the distinguishing feature. Fissures are especially common during periods of constipation, after childbirth, or with chronic diarrhea.
Infections That Cause Bloody Stool
Bacterial infections from organisms like Salmonella, E. coli, and Campylobacter can inflame the lining of the colon enough to cause bloody diarrhea. This type of bleeding is usually accompanied by cramping, fever, nausea, and frequent loose stools. It often starts a day or several days after eating contaminated food or water.
If your bloody stool came on suddenly alongside diarrhea and stomach cramps, especially after traveling, eating something questionable, or being around others who are sick, an infection is a likely explanation. Most cases resolve within a week, but bloody diarrhea combined with high fever or an inability to keep fluids down needs medical attention sooner rather than later.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease cause chronic inflammation in the digestive tract that can lead to recurring bloody stools. If you’ve been dealing with bloody diarrhea on and off for weeks, along with abdominal pain, urgency, or unexplained weight loss, inflammatory bowel disease is worth investigating. These conditions tend to flare and remit, so symptoms may come and go. They’re typically diagnosed through colonoscopy and biopsy.
Diverticular Bleeding
Diverticula are small pouches that form in the walls of the colon, most commonly in people over 40. They’re usually harmless, but occasionally a blood vessel near one of these pouches can rupture and produce a sudden, large amount of bright red or maroon blood. Diverticular bleeding is often painless, which makes it particularly startling. It’s one of the most common causes of significant lower GI bleeding in older adults. The bleeding usually stops on its own, but the volume alone often sends people to the emergency room.
Medications That Increase Bleeding Risk
Certain medications make GI bleeding more likely by thinning the blood or irritating the digestive lining. Prescription blood thinners (anticoagulants) roughly quadruple the risk of gastrointestinal bleeding. Even low-dose aspirin doubles the risk. Over-the-counter pain relievers like ibuprofen and naproxen increase the risk of upper GI bleeding by about 2.5 times. If you’re taking any of these regularly and notice blood in your stool, that medication is a strong candidate for what’s going on, and it’s worth discussing with your doctor promptly.
Foods and Supplements That Mimic Blood
Before assuming the worst, consider what you’ve eaten recently. Beets, tomatoes, red gelatin, and certain red-dyed foods can turn stool a reddish color that looks convincingly like blood. Iron supplements and bismuth (the active ingredient in some stomach medications) can turn stool very dark or black, mimicking the appearance of digested blood. If you ate a large beet salad yesterday and your stool looks alarming today, that may be the entire explanation. The difference is that actual blood in stool tends to be streaky or mixed in, while food-related color changes are more uniform.
Colorectal Cancer as a Cause
This is what most people are really worried about when they search this question. Colorectal cancer can cause rectal bleeding, and it’s important to take the symptom seriously. A study from the University of Louisville analyzing patients under 50 who underwent colonoscopy found that rectal bleeding was the strongest predictor of colorectal cancer in that age group, increasing the odds of diagnosis by 8.5 times compared to those without bleeding. Among patients ultimately diagnosed with early-onset colorectal cancer, 88% had undergone their colonoscopy because of symptoms like bleeding.
That said, context matters. A one-time episode of bright red blood on the toilet paper in a 25-year-old with a history of constipation is a very different situation from persistent bleeding with weight loss, changes in stool shape, or fatigue. Red flags that raise concern for cancer include bleeding that persists for more than a few weeks, a change in bowel habits (new constipation or narrower stools), unintentional weight loss, and a feeling that your bowel doesn’t fully empty. The current recommended age to begin routine colorectal cancer screening is 45, but symptoms like unexplained rectal bleeding justify a colonoscopy at any age.
When Bloody Stool Is an Emergency
Most rectal bleeding is not an emergency. A small amount of bright red blood that happens once or twice and then stops can usually wait for a scheduled doctor’s visit. But certain situations require immediate care.
Call 911 or get to an emergency room if you’re experiencing heavy or continuous bleeding along with any signs that your body isn’t coping well: rapid or shallow breathing, dizziness or lightheadedness when you stand up, blurred vision, fainting, confusion, nausea, cold or clammy skin, or very little urine output. These are signs of significant blood loss. You should also seek emergency care if heavy rectal bleeding is accompanied by severe abdominal pain or cramping, regardless of other symptoms.
For bleeding that’s mild, one-time, and not accompanied by any of those warning signs, it’s still worth bringing up with your doctor, especially if you’re over 40, have a family history of colorectal cancer, or notice it happening more than once. A brief evaluation can rule out serious causes and give you a clear answer.