Bloody stool has dozens of possible causes, and most of them are not life-threatening. Hemorrhoids and small tears near the anus account for the majority of cases people notice at home. But the same symptom can also signal inflammatory bowel disease, diverticular bleeding, infections, medication side effects, and, less commonly, colorectal cancer. The color, volume, and accompanying symptoms tell you a lot about where the bleeding is coming from and how urgently you need to act.
What the Blood Looks Like Matters
Bright red blood on the toilet paper or coating the outside of a stool almost always comes from somewhere near the end of the digestive tract: the rectum, the anus, or the lower colon. This is the type most people encounter, and it is frequently harmless. Dark, tarry, foul-smelling stool, called melena, points to bleeding higher up, often in the stomach or duodenum. An older estimate found that roughly 95 percent of bleeding episodes that show up as melena or vomiting blood originate in the stomach or duodenum, with duodenal ulcers alone responsible for about 40 percent of those cases.1Mayo Clinic Proceedings. Tumor of the Small Intestine as the Cause of Recurrent Melena: Report of Two Cases Maroon or burgundy-colored stool tends to suggest a source somewhere in between, such as the small intestine or right colon.
This color guide is not absolute. A large, brisk bleed high in the GI tract can move through so quickly that it comes out bright red. And certain foods, iron supplements, and bismuth-containing antacids can turn stool dark without any bleeding at all. Still, paying attention to color gives you and your doctor a useful starting point.
Hemorrhoids and Anal Fissures
The single most common reason people see bright red blood after a bowel movement is hemorrhoids, swollen blood vessels inside or just outside the anus. They are often painless, especially internal ones, and the bleeding typically stops on its own. External hemorrhoids can hurt, itch, and sometimes form a hard clot that feels like a marble under the skin.
Anal fissures, small tears in the lining of the anal canal, are the other frequent culprit. They tend to cause a sharp, stinging pain during a bowel movement along with a streak of blood. In the United States alone, about 342,000 new cases of anal fissure disease are reported each year, and they most often affect younger and middle-aged adults.2JAMA. Diagnosis and Treatment of Anal Fissures in 2021 Most acute fissures heal within a few weeks with increased fiber, more water, and warm baths. A fissure that persists beyond about eight weeks is considered chronic and may need medicated ointments or, occasionally, a minor procedure.
Diverticular Bleeding
Diverticula are small pouches that bulge outward through weak spots in the colon wall. They become increasingly common with age and usually cause no trouble. When one of these pouches bleeds, though, the result can be dramatic: a sudden, painless gush of dark red or maroon blood. It is one of the more common causes of significant lower GI bleeding in older adults.
The bleeding happens because blood vessels running along the wall of a diverticulum lack the muscular protection they would have in normal colon tissue. Chronic friction from stool and the colon’s own squeezing motion causes small injuries and thickening of vessel walls over time, making them stiff and prone to rupture.3PubMed Central. Colonic diverticular hemorrhage: Etiology, diagnostic challenges, and evolving therapeutic strategies Studies of these bleeding vessels show lopsided thickening and scarring right at the dome of the pouch, consistent with repeated mechanical trauma from inside the colon.4PubMed. Pathogenesis of bleeding colonic diverticulosis Despite how alarming it looks, diverticular bleeding stops on its own in most cases, though it can recur.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease are the two main forms of inflammatory bowel disease. Both can cause bloody diarrhea, but they behave differently. Ulcerative colitis is confined to the colon and involves only the surface lining, producing rectal bleeding, diarrhea, and abdominal pain as its hallmark symptoms.5PubMed. Diagnosis and classification of ulcerative colitis Crohn’s disease can affect any part of the digestive tract and penetrates deeper into the bowel wall, so its bleeding patterns are less predictable.
If you are having recurrent bloody diarrhea, especially with cramping, urgency, and weight loss that last more than a couple of weeks, inflammatory bowel disease belongs on the list of things to rule out. These are chronic conditions that wax and wane, and early diagnosis matters because sustained inflammation raises long-term colorectal cancer risk.
Infections
A gut infection can cause bloody diarrhea that comes on quickly. Bacteria such as Shigella, Salmonella, and Campylobacter are common culprits, as is the protozoan parasite Entamoeba histolytica, which often brings fever and a more toxic presentation.6Semantic Scholar. Infectious Diseases Dysentery (Infectious Diarrhea) — Causes and Treatment Travelers to regions with poor sanitation and people who eat undercooked meat or contaminated produce are at higher risk. Most bacterial dysentery resolves within a week, but severe dehydration or high fever warrants prompt medical attention.
Medications That Raise Bleeding Risk
Several widely used drug classes make GI bleeding more likely, and you may not immediately connect the dots between a daily pill and blood in your stool. Anticoagulants (blood thinners like warfarin and the newer direct oral anticoagulants) carry the highest risk, roughly quadrupling the chance of both upper and lower GI bleeding. Low-dose aspirin and other antiplatelet drugs roughly double it.7PubMed. Risk of upper and lower gastrointestinal bleeding in patients taking nonsteroidal anti-inflammatory drugs, antiplatelet agents, or anticoagulants NSAIDs like ibuprofen and naproxen also increase risk, with a stronger effect on upper GI bleeds (around the stomach and duodenum) than lower ones.
Less well known is that SSRIs, a common type of antidepressant, and corticosteroids are also associated with GI bleeding. One study found that SSRI use was linked to almost four times the odds of non-ulcer GI bleeds, and corticosteroids nearly three times the odds, compared to people not taking those drugs.8PubMed. Impact of aspirin, NSAIDs, warfarin, corticosteroids and SSRIs on the site and outcome of non-variceal upper and lower gastrointestinal bleeding If you are on any of these medications and notice blood in your stool, mention both the symptom and your medication list when you call your doctor.
Ischemic Colitis
When blood flow to a part of the colon drops sharply, the tissue can become damaged and bleed. This is ischemic colitis, and it tends to strike older adults, particularly those with heart disease, low blood pressure, or who have recently had surgery. The colon has a few natural weak spots, called watershed areas, where the blood supply is thinnest. These include the splenic flexure (a bend in the upper left colon), the junction of the sigmoid colon and rectum, and the area near the cecum.9PubMed Central. Ischemic bowel disease in 2021 10PubMed. Ischemic colitis arising in watershed areas of the colonic blood supply: a report of two cases
Symptoms usually include sudden crampy abdominal pain on the left side, followed by bloody diarrhea within hours. Most cases are mild and resolve with bowel rest and fluids, but severe ischemia can lead to tissue death and require surgery. If you are an older adult with vascular disease and develop sudden left-sided abdominal pain with bloody stool, treat it as urgent.
Colorectal Cancer and Polyps
This is the possibility that makes people most anxious, and it is worth putting into perspective. Colorectal cancer is a real but relatively uncommon cause of rectal bleeding compared to hemorrhoids and fissures. Still, it is exactly the scenario where ignoring the symptom has the highest cost. Tumors and large polyps bleed because they outgrow their blood supply or erode into surrounding tissue. The blood may be visible or hidden (occult), and the bleeding can be intermittent enough to be easy to dismiss.
Interestingly, when colorectal cancer does present with visible blood in the stool, the tumors tend to have features associated with a better prognosis. One study found that tumors discovered because of visible rectal bleeding were more often lower-grade, had a pattern of pushing borders rather than infiltrating borders, and showed stronger immune responses in the surrounding tissue. Patients whose cancer was found this way had better survival outcomes.11APMIS. Colon cancer: association of histopathological parameters and patients’ survival with clinical presentation The takeaway is not that bloody stool is a “good sign” in cancer; it is that visible bleeding prompts earlier investigation, and earlier investigation catches cancers at more treatable stages.
Vascular Malformations in Older Adults
Angiodysplasia refers to fragile, abnormally formed blood vessels in the wall of the colon or elsewhere in the gut. These tend to develop with aging and are a recognized cause of both visible and hidden GI bleeding, particularly in people over 60.12PubMed. Gastrointestinal angiodysplasia – pathogenesis, diagnosis and management The bleeding from angiodysplasia is often slow and chronic, leading to iron-deficiency anemia that gets picked up on routine bloodwork before anyone notices blood in the stool. In some cases, though, it can cause sudden, brisk bleeding that needs endoscopic treatment.
Causes Specific to Children
When a child passes bloody stool, the differential diagnosis shifts. While anal fissures and infections are still common pediatric causes, a few conditions are particular to children. Meckel’s diverticulum, a leftover pouch from fetal development found in the small intestine, classically shows up as painless lower GI bleeding in a young child without signs of infection or inflammatory bowel disease.13PubMed Central. Not Just Painless Bleeding: Meckel’s Diverticulum as a Cause of Small Bowel Obstruction in Children—Two Cases and a Review of the Literature Intussusception, where one segment of intestine telescopes into another, causes the classic “currant jelly” stool mixed with mucus and blood, along with episodes of severe abdominal pain. Milk protein allergy in infants can also cause streaks of blood in stool, usually alongside fussiness and loose stools. In children, bloody stool deserves prompt pediatric evaluation even when the child seems otherwise well.
Bleeding After a Colonoscopy or Polypectomy
If you have recently had polyps removed during a colonoscopy, some bleeding in the days afterward is a known possibility. In one study, post-polypectomy bleeding occurred in under 2 percent of patients, with most of it happening right away and delayed bleeding (days later) occurring in roughly half a percent of cases.14PubMed. Risk factors for post-polypectomy bleeding; a retrospective case-control study of a high-volume colonoscopy center Larger polyps, polyps on stalks, polyps located near the ileocecal valve, and having more polyps removed all increase the risk. Delayed bleeding tends to happen more often with polyps in the right colon, where the thinner bowel wall and wider lumen may reduce the effectiveness of natural compression that normally helps seal small wounds.15Annals of Coloproctology. Analysis of Delayed Postpolypectomy Bleeding in a Colorectal Clinic A small amount of blood after polypectomy usually resolves on its own, but persistent or heavy bleeding after the procedure warrants a call to your gastroenterologist.
When to Worry
Not every episode of rectal bleeding requires a trip to the emergency department, but some scenarios call for immediate action. You should seek urgent care if you experience:
- Heavy volume: blood filling the toilet bowl, large clots, or bleeding that does not stop after several minutes.
- Hemodynamic signs: dizziness, lightheadedness, rapid heartbeat, or feeling faint, which suggest enough blood loss to affect your circulation.
- Black, tarry stool: melena points to an upper GI source and usually means a significant amount of blood has entered the digestive tract.
- Severe abdominal pain: particularly sudden onset, which could indicate ischemia, perforation, or another surgical emergency.
- Fever with bloody diarrhea: suggests a serious infection or a flare of inflammatory bowel disease.
For less dramatic bleeding, such as small amounts of bright red blood on the paper after a hard stool in an otherwise healthy younger person, it is reasonable to try a few days of increased fiber and hydration before calling a doctor. But bleeding that recurs over weeks, bleeding accompanied by a change in bowel habits, unexplained weight loss, or new fatigue, even without alarming volume, warrants investigation. These are the patterns that sometimes hide a polyp or early cancer behind what feels like “just hemorrhoids.”
Emergency physicians and gastroenterologists sometimes use formal risk scores to decide who can safely go home and who needs to be admitted. The Oakland score, developed and validated for lower GI bleeding, uses factors like age, sex, prior bleeding episodes, heart rate, blood pressure, rectal exam findings, and hemoglobin levels. A score of 8 or lower predicts a 95 percent chance that the patient can be safely managed as an outpatient.16JAMA Network Open. Comparison of Risk Scores for Lower Gastrointestinal Bleeding: A Systematic Review and Meta-analysis 17The Lancet Gastroenterology & Hepatology. Derivation and external validation of an early warning score for acute lower gastrointestinal bleeding: an observational cohort study You probably will not calculate this score yourself, but it helps to know that doctors have structured tools for separating “safe to watch” from “needs to stay.”
When the source of bleeding is not clear from the initial presentation, a colonoscopy is the standard next step for suspected lower GI bleeding. If a patient is hemodynamically unstable or the bleeding seems brisk enough to come from higher up, an upper endoscopy is often done first.18Gastrointestinal Endoscopy Clinics. Colonoscopy in Acute Lower Gastrointestinal Bleeding: Diagnosis, Timing, and Bowel Preparation
Stool Tests for Hidden Blood
Sometimes blood in the stool is invisible to the naked eye. This is called occult blood, and detecting it is the basis for widely used colorectal cancer screening programs. Two main types of stool tests exist: the older guaiac-based test (gFOBT) and the newer immunochemical test (FIT). FIT is consistently more sensitive. A large Cochrane review found that FIT detected roughly twice as many cases of significant colorectal growths as the guaiac test, with similar rates of false positives.19PubMed Central. Guaiac-based faecal occult blood tests versus faecal immunochemical tests for colorectal cancer screening in average-risk individuals A separate study found that FIT’s sensitivity for significant growths was about 61 percent compared to about 24 percent for the guaiac test.20PubMed. Diagnostic accuracy of immunochemical versus guaiac faecal occult blood tests for colorectal cancer screening Neither test is perfect, and a single negative result does not rule out cancer, which is why screening programs recommend repeating the test annually or biennially. A positive FIT always means further investigation, usually a colonoscopy, even if you feel fine.
Why People Put Off Getting Checked
Rectal bleeding is common enough that many people experience it and never mention it to a doctor. Research on this delay shows a pattern: people who are not particularly worried by the bleeding, who do not routinely look at their stool, or who try home remedies first are significantly more likely to wait more than two weeks before seeking care.21PubMed. Rectal bleeding. Patient delay in presentation One population-based survey found that embarrassment was a straightforward barrier for some people, with respondents admitting they could not bring themselves to raise the topic during a doctor visit even when they had the opportunity.22BMJ. Rectal bleeding: prevalence and consultation behaviour Fear of what the doctor might find, or of the procedures involved, adds another layer of avoidance.23PLoS ONE. Patient Delay in Colorectal Cancer Patients: Associations with Rectal Bleeding and Thoughts about Cancer
The irony is that the conditions most worth catching early are the same ones people are most afraid of. And the conditions people are least afraid of, like hemorrhoids, are the ones that make it easy to rationalize skipping the call. If you have been bleeding on and off for weeks and telling yourself it is probably nothing, that is exactly the scenario where making an appointment matters most. It probably is nothing serious, but confirming that takes less time and discomfort than most people imagine.