The most common reason for blood in your stool is hemorrhoids, which are swollen veins in your rectum or anus. Anal fissures (small tears in the skin around the anus) are the second most likely cause. While these two conditions account for the majority of cases, blood in your stool can also signal something more serious, so the color, amount, and any accompanying symptoms all matter.
What the Color of the Blood Tells You
The appearance of blood in your stool is a useful clue about where the bleeding is coming from. Bright red blood on toilet paper or in the bowl typically points to a source near the end of the digestive tract: the rectum or anus. This is the most common scenario and often the least concerning.
Dark maroon or wine-colored blood usually means the bleeding is higher up, in the colon or small intestine. Black, tarry, sticky stools suggest the source is even further upstream, in the stomach or upper small intestine. Blood turns black as it’s digested on its way through the gut, and it takes roughly 100 to 200 mL of blood in the upper digestive tract to produce that tarry appearance. Black stools can persist for several days after the bleeding has actually stopped.
One important caveat: not everything that looks like blood actually is. Beets, tomatoes, and foods with red dye can make stool appear reddish. On the dark end, blueberries, black licorice, iron supplements, bismuth (the active ingredient in Pepto-Bismol), and activated charcoal can all turn stool black without any bleeding involved.
Hemorrhoids
Hemorrhoids are swollen veins in the anus or rectum, and they’re the single most common cause of rectal bleeding. You might notice bright red blood on the toilet paper, dripping into the bowl, or coating the surface of your stool. Most hemorrhoids don’t actually hurt. You may feel itching, mild discomfort, or notice a small lump near the anus, but pain is often minimal unless a clot forms inside the hemorrhoid.
Straining during bowel movements, sitting for long periods, pregnancy, and chronic constipation all increase your risk. In most cases, hemorrhoids resolve with simple changes: more fiber, more water, and avoiding prolonged straining on the toilet. Over-the-counter creams and sitz baths (sitting in a few inches of warm water) can help with discomfort.
Anal Fissures
An anal fissure is a small tear in the lining of the anal canal, usually caused by passing a hard or large stool. Unlike hemorrhoids, fissures tend to hurt, sometimes significantly. The pain is sharpest during a bowel movement and can linger for hours afterward as a burning or throbbing sensation. You’ll typically see bright red blood on the toilet paper or on the surface of the stool.
Most fissures heal on their own within a few weeks if you keep stools soft. Increasing fiber and fluid intake is the first step. Warm baths can relax the muscle around the anus and promote healing. Fissures that don’t heal after several weeks may need prescription treatment to relax the anal sphincter.
Diverticular Bleeding
Diverticulosis, a condition where small pouches form in the wall of the colon, becomes increasingly common after age 40. These pouches develop at weak spots where blood vessels penetrate the colon wall. As a pouch pushes outward, the blood vessels draped over it become exposed and vulnerable to injury.
When one of these vessels ruptures, the result is often a sudden, painless gush of dark red or maroon blood. It can be alarming because of the volume, but the bleeding stops on its own in about 80% of cases. If you experience a large amount of painless bleeding, diverticular disease is one of the more likely explanations, particularly if you’re over 50.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are chronic conditions where the immune system attacks the lining of the digestive tract, creating inflammation and ulcers that bleed. Blood in the stool from these conditions is rarely the only symptom. You’ll typically also experience some combination of persistent diarrhea (sometimes with mucus or pus), abdominal cramping, urgency to use the bathroom, fatigue, weight loss, fever, or loss of appetite.
Ulcerative colitis affects the colon and rectum, causing inflammation in the surface lining. Crohn’s disease can affect any part of the digestive tract and often involves deeper layers of tissue. Both conditions tend to flare and remit over time. If you’re having bloody diarrhea along with abdominal pain, cramping, or unexplained weight loss, these conditions warrant evaluation.
Upper GI Bleeding
Peptic ulcers, which are open sores in the stomach lining or the upper part of the small intestine, can cause blood to appear in your stool. Because the blood travels through the entire digestive tract before exiting, it’s been broken down by digestive enzymes and bacteria along the way. The result is stool that looks black, tarry, and unusually sticky, with a distinctly foul smell.
Severe ulcer bleeding can also cause vomiting blood (which may look like coffee grounds), dizziness, and weakness. Ulcers are most commonly caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen or aspirin.
Colorectal Cancer
Colorectal cancer is a less common but serious cause of blood in the stool. The bleeding is often subtle, sometimes invisible to the naked eye, and may develop gradually alongside changes in bowel habits, unexplained weight loss, or a feeling that your bowel doesn’t empty completely. The USPSTF recommends colorectal cancer screening starting at age 45 for average-risk adults, with colonoscopy every 10 years if results are negative.
Rectal bleeding alone, without other risk factors, is far more likely to be hemorrhoids or a fissure than cancer. But persistent bleeding, especially if it’s new and you’re over 45 or have a family history of colorectal cancer, is worth getting checked.
When Rectal Bleeding Is an Emergency
Most rectal bleeding is minor and stops on its own. But heavy or continuous bleeding, or bleeding paired with severe abdominal pain, calls for an emergency room visit. Call 911 if you have significant rectal bleeding along with any signs that your body isn’t coping with the blood loss: rapid or shallow breathing, dizziness when standing, blurred vision, fainting, confusion, nausea, cold or clammy skin, or very low urine output. These are signs of shock, and they require immediate treatment.
How Doctors Figure Out the Cause
Your doctor will start by asking about the color, frequency, and amount of blood, plus any other symptoms like pain, changes in bowel habits, or weight loss. A physical exam of the anal area can often identify hemorrhoids or fissures right away.
If the source isn’t obvious, the next step is usually a stool test that detects hidden blood, or a colonoscopy to visually inspect the lining of the colon. A colonoscopy can identify polyps, inflammation, diverticular disease, or tumors. For suspected upper GI bleeding (black tarry stools), an upper endoscopy, where a camera is passed through the mouth into the stomach, is typically used instead.
A single episode of bright red blood on the toilet paper after straining doesn’t necessarily require a full workup. But recurring bleeding, changing stool color, or any accompanying symptoms like pain, weight loss, or fatigue should prompt a visit to your doctor so the cause can be identified and treated.