Blood in diarrhea can come from infections, inflammatory conditions, medication side effects, or structural problems anywhere along the digestive tract. The color of the blood is one of the most useful clues to where the bleeding originates: bright red blood typically points to the lower intestine or rectum, while dark, tarry stools suggest bleeding higher up in the digestive system, such as the stomach or small intestine. Roughly 100 to 200 mL of blood in the upper digestive tract is needed to produce those dark stools, and they can persist for days after the bleeding has actually stopped.
What Blood Color Tells You
Bright red blood on or mixed into diarrhea usually means the source is in the colon or rectum. This is the most common presentation when people notice blood in loose stools, and it can range from a few streaks to stool that looks mostly red or maroon.
Black, sticky, tar-like stools indicate that blood has traveled through the digestive system long enough for stomach acid to break it down, turning red blood cells dark brown or black. This typically means the bleeding started in the stomach or upper small intestine. One important caveat: iron supplements, bismuth (the active ingredient in some antacids), and certain dark foods can also turn stool black without any actual bleeding.
Bacterial and Parasitic Infections
Infections are one of the most common reasons for sudden bloody diarrhea. Certain strains of E. coli, particularly Shiga toxin-producing E. coli (STEC, including the well-known O157:H7), cause bloody diarrhea along with severe stomach cramps and vomiting. These bacteria produce toxins that damage the lining of the intestine directly. Another type, enteroinvasive E. coli, invades the intestinal wall and can cause watery diarrhea that sometimes turns bloody, often accompanied by fever.
Shigella and Campylobacter are two other common culprits. Both invade the intestinal lining, triggering inflammation and ulceration that leads to visible blood and mucus in stool. These infections are usually picked up through contaminated food, water, or close contact with an infected person. Most cases resolve within a week, though severe infections sometimes need antibiotics.
A parasitic infection called amebiasis, caused by Entamoeba histolytica, can also produce bloody diarrhea with stomach pain and fever. Interestingly, only about 10% to 20% of people infected with this parasite actually become ill. Diagnosing it can be tricky because other harmless parasites look nearly identical under a microscope, and stool samples from multiple days are often needed to detect it.
Inflammatory Bowel Disease
Two chronic conditions, ulcerative colitis and Crohn’s disease, cause the immune system to attack the digestive tract, leading to ongoing inflammation that can produce bloody diarrhea over weeks or months rather than days.
Ulcerative colitis is the more likely of the two to cause visible blood. It affects only the colon and always starts at the rectum, so bloody diarrhea, urgent trips to the bathroom, and a persistent feeling of incomplete emptying are hallmark symptoms. Cramping and bleeding tend to center in the lower abdomen.
Crohn’s disease can affect any part of the digestive tract, including the small intestine. When it does involve the small bowel, diarrhea is often nonbloody, and the more prominent symptoms are abdominal pain and unintended weight loss. Blood is more likely when Crohn’s affects the colon. If your doctor suspects inflammatory bowel disease, a colonoscopy is the standard next step for confirming the diagnosis.
Medications That Increase Bleeding Risk
Several common medications can damage the lining of the digestive tract or make existing bleeding worse. NSAIDs like ibuprofen, naproxen, and diclofenac are among the biggest offenders. These pain relievers reduce the protective mucus layer that shields the intestinal wall, leaving it vulnerable to erosion and ulceration. A large multicenter study found that NSAID use was an independent risk factor for bleeding in the middle portion of the digestive tract.
Blood thinners, including warfarin and newer anticoagulants, don’t cause the damage themselves but can turn minor bleeding that would otherwise go unnoticed into something visible. The same study identified anticoagulants and certain antiplatelet drugs (used after heart procedures or strokes) as independent risk factors for intestinal bleeding. If you take any of these medications and notice blood in your stool, that’s worth reporting to your doctor promptly.
Surprisingly, proton pump inhibitors, commonly prescribed to protect the stomach, were also linked to increased bleeding in the small intestine. The likely mechanism involves changes to gut bacteria: suppressing stomach acid allows bacterial overgrowth, which can then damage the small bowel lining.
Ischemic Colitis
When blood flow to part of the colon is temporarily reduced, the tissue becomes inflamed and can bleed. This is called ischemic colitis, and it happens mostly in adults older than 60, with women affected more often than men. Symptoms include sudden crampy abdominal pain, diarrhea, and bright red or maroon blood in the stool (sometimes blood alone without stool). The condition can be difficult to diagnose because it mimics other digestive problems. Imaging with CT scans or colonoscopy is typically needed to confirm it.
Hemorrhoids and Anal Fissures
Not all blood in diarrhea comes from deep inside the intestine. Hemorrhoids and anal fissures, both located at or near the opening of the rectum, can bleed during episodes of diarrhea. Chronic diarrhea is actually a primary cause of anal fissures because the repeated passage of loose stool irritates and tears the delicate anal lining. Both conditions produce fresh, bright red blood that you might see on the stool surface, on toilet paper, or in the bowl. Pain, itching, and a stinging sensation during bowel movements are common with fissures, while hemorrhoids may cause bleeding without much pain at all.
Causes in Infants and Young Children
In babies, one of the more serious causes of bloody stool is intussusception, a condition where one section of intestine telescopes into the section next to it, cutting off blood supply. The first sign is usually sudden, loud crying from belly pain, with the baby pulling their knees to their chest. These painful episodes come and go, typically every 15 to 20 minutes at first, then grow longer and more frequent. The stool can become mixed with blood and mucus, sometimes described as having a “currant jelly” appearance. Vomiting, a lump in the belly, and weakness or lack of energy are other signs. Not every child shows all these symptoms, and some older children have only pain. Intussusception requires emergency treatment.
Cow’s milk protein allergy is another common cause of blood-streaked stools in infants, particularly in formula-fed babies or breastfed babies whose mothers consume dairy. It typically presents in the first few months of life and resolves when the offending protein is removed from the diet.
When Bloody Diarrhea Is an Emergency
Some episodes of bloody diarrhea resolve on their own, particularly mild infections or flare-ups of hemorrhoids. But certain signs indicate you need emergency care: passing large amounts of blood, feeling lightheaded or dizzy, a noticeably rapid heart rate, or feeling weak. These suggest significant blood loss that may need urgent treatment. Bloody diarrhea with high fever can also signal a severe infection that needs immediate attention, especially in young children, older adults, or anyone with a weakened immune system.
How Doctors Find the Cause
The diagnostic approach depends on what your doctor suspects based on your symptoms, age, and medical history. For suspected infections, particularly after recent travel or exposure to contaminated food, stool cultures are the first step to identify the specific bacteria or parasite. For suspected inflammatory bowel disease, which typically presents with recurring bloody diarrhea, abdominal pain, and weight loss over time, colonoscopy is the standard diagnostic tool. Most patients with significant lower intestinal bleeding will eventually undergo colonoscopy once they’re stable, since it allows doctors to both identify and sometimes treat the bleeding source directly. In cases where someone is too unstable for that procedure, CT imaging with contrast dye can help locate where the bleeding is coming from.