Bloody diarrhea is most commonly caused by bacterial infections, but it can also result from inflammatory bowel disease, reduced blood flow to the colon, medications, and several other conditions. The color and pattern of the blood offer important clues: bright red blood typically points to a problem in the colon or rectum, while black, tarry stools suggest bleeding higher up in the digestive tract, such as the stomach or small intestine.
How Blood Color Points to the Source
The appearance of blood in your stool is one of the first things that helps narrow down the cause. Bright red blood passed with or in the stool, called hematochezia, usually comes from the colon or rectum. Black, tarry stools, known as melena, typically signal bleeding in the upper digestive tract. Blood from the stomach or small intestine turns dark as it’s digested on its way through.
These rules aren’t absolute. If stool moves slowly through the intestines, even bleeding from the lower end of the small intestine can produce dark stools. And very rapid bleeding from a stomach ulcer can produce bright red blood if it moves through the system fast enough. Still, blood color is a reliable starting point for understanding where the problem is.
Bacterial Infections
Foodborne bacteria are among the most common causes of bloody diarrhea in otherwise healthy people. The usual culprits include certain strains of E. coli, Salmonella, Shigella, and Campylobacter. These infections typically start one to three days after eating contaminated food or drinking contaminated water, and most resolve within a week.
Some strains of E. coli are particularly dangerous. Shiga toxin-producing E. coli (often linked to undercooked ground beef or contaminated produce) releases toxins that bind to the cells lining your intestine. Once inside, these toxins shut down the cell’s ability to make proteins, killing the cell. As intestinal cells die, the lining breaks down and bleeds. If the toxin enters the bloodstream, it can damage small blood vessels in the kidneys, leading to a serious complication called hemolytic uremic syndrome. This is especially concerning in young children and older adults.
Parasitic infections like amoebic dysentery can also cause bloody diarrhea, particularly after travel to regions with limited sanitation.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are chronic conditions where the immune system attacks the lining of the digestive tract, creating ulcers that bleed. Ulcerative colitis affects the colon and rectum and frequently causes bloody diarrhea as one of its earliest symptoms. Crohn’s disease can affect any part of the digestive tract but most often involves the end of the small intestine.
Unlike infections, inflammatory bowel disease doesn’t resolve on its own. Symptoms tend to come in flares, with periods of remission in between. Other signs include persistent abdominal cramping, urgency to use the bathroom, weight loss, and fatigue. Bloody diarrhea lasting more than a few days, especially if it recurs, raises the possibility of inflammatory bowel disease and typically leads to a colonoscopy for diagnosis.
Ischemic Colitis
When blood flow to part of the colon drops, the tissue becomes oxygen-starved, swollen, and damaged. This is ischemic colitis, and it mostly affects adults over 60. Atherosclerosis (fatty buildup in artery walls) is a major contributor, but episodes can also be triggered by low blood pressure from dehydration, heart failure, or shock.
Risk factors include high cholesterol, diabetes, rheumatoid arthritis, sickle cell anemia, and conditions that cause blood vessel inflammation. The typical pattern is sudden cramping on the left side of the abdomen followed by bloody diarrhea within 24 hours. Most mild cases heal on their own with supportive care, but severe cases where a section of the colon loses its blood supply entirely can require surgery.
Medications, Especially NSAIDs
Non-steroidal anti-inflammatory drugs like ibuprofen, naproxen, and aspirin are a well-documented cause of gastrointestinal bleeding. A landmark study in The Lancet found that current NSAID use nearly quintupled the risk of upper gastrointestinal bleeding compared to non-use. The risk climbed steeply with dose: low doses roughly doubled the risk, while high doses increased it more than eightfold.
Not all NSAIDs carry equal risk. Ibuprofen had the lowest bleeding risk among common options (about double the baseline), while drugs like piroxicam and naproxen carried considerably higher risks. NSAIDs damage the stomach lining by suppressing the protective mucus layer, making it vulnerable to acid erosion. Blood thinners, corticosteroids, and certain antidepressants can also increase bleeding risk, particularly when combined with NSAIDs.
Diverticular Bleeding
Diverticulosis, where small pouches form along the colon wall, is extremely common after age 40. Most people never know they have it. But in 3 to 5% of cases, a blood vessel near one of these pouches ruptures, causing sudden, painless, and sometimes heavy rectal bleeding. The blood is usually dark red or maroon.
Diverticular bleeding is one of the most common causes of significant lower gastrointestinal bleeding in older adults. It often stops on its own, but the volume of blood can be alarming. Colonoscopy is the first-line tool for both identifying the bleeding source and treating it.
Hemorrhoids and Anal Fissures
These are worth distinguishing from true bloody diarrhea because they’re so common and often mistaken for something more serious. Internal hemorrhoids cause painless bright red blood that you’ll notice on toilet paper, in the bowl, or coating the stool. The blood sits on the surface rather than being mixed in. Pain is rare unless a hemorrhoid becomes strangulated.
Anal fissures, which are small tears in the lining of the anus, cause sharp pain during and for hours after a bowel movement, along with bright red bleeding. Both conditions are triggered by straining, hard stools, or chronic diarrhea. If you’re seeing blood mixed into loose stools rather than on the surface, the source is more likely higher up in the colon.
Colorectal Cancer and Polyps
Blood in the stool can be a sign of colorectal cancer or precancerous polyps, which is why this symptom should never be dismissed as “just hemorrhoids” without evaluation, particularly in people over 45 or those with a family history. Polyps and early-stage cancers often bleed intermittently and in small amounts, sometimes producing blood invisible to the naked eye. As tumors grow, bleeding becomes more noticeable and may be accompanied by changes in bowel habits, unexplained weight loss, or a feeling that the bowel doesn’t empty completely.
Routine colorectal cancer screening is recommended starting at age 45 for people at average risk. But screening guidelines apply to people without symptoms. If you’re actively seeing blood in your stool, that’s not a screening situation. It’s a reason to get a diagnostic evaluation regardless of your age.
Bloody Diarrhea in Infants and Children
In babies between 5 months and 3 years old, one cause that demands immediate attention is intussusception, where one section of the intestine telescopes into the next. The hallmark is sudden, intense crying from abdominal pain that comes and goes. Babies often draw their knees to their chest during episodes. A distinctive “currant jelly” stool, a mix of blood and mucus, is a classic sign. Fever, vomiting, and eventually lethargy or pale skin can follow. This is a medical emergency.
Cow’s milk protein allergy is a more common and less urgent cause of bloody stool in infants, typically appearing in the first few months of life. Bacterial infections and, in older children, inflammatory bowel disease are also possibilities.
When Bloody Diarrhea Is an Emergency
Some episodes of bloody diarrhea resolve on their own, particularly mild food poisoning. But certain signs indicate you need emergency care: large amounts of blood, lightheadedness or dizziness, a rapid heart rate, weakness, or feeling faint. These suggest significant blood loss. Bloody diarrhea accompanied by high fever, severe abdominal pain, or confusion also warrants urgent evaluation. In young children, signs of shock (pale skin, lethargy, sweating) alongside bloody stool require immediate medical attention.