Passing blood clots during a bowel movement usually means there is active bleeding somewhere in the gastrointestinal tract, most often in the colon or rectum. The clots form when blood pools in the intestine long enough to coagulate before being expelled. The most common culprits are hemorrhoids, diverticular disease, and small abnormal blood vessels in the colon wall, but the list of possible causes runs from the trivially benign to the genuinely urgent. What matters most is figuring out where you fall on that spectrum, and that depends on a handful of details about the bleeding itself, your age, and what else is going on in your body.
The Most Likely Causes
For most people, bloody stool with clots traces back to one of a few common problems in the lower digestive tract. Internal hemorrhoids are probably the single most frequent source of bright red rectal bleeding in adults. Swollen blood vessels inside the rectum can ooze or drip blood, and if the blood sits in the rectum before your next bowel movement, it can clot. A study of patients treated for bleeding internal hemorrhoids found that the problem affected people across a wide age range, from their early twenties to late seventies.1SpringerLink / Tech Coloproctol. Superior rectal artery embolization for bleeding internal hemorrhoids Hemorrhoid bleeding tends to be painless, bright red, and noticed on toilet paper or in the bowl rather than mixed into the stool itself.
Diverticular bleeding is another major source, and it is particularly known for producing large-volume bleeding with clots. Diverticula are small pouches that bulge outward from the colon wall. Fewer than one in twenty people with diverticulosis ever bleed from one, but when they do, the bleeding can be dramatic: sudden, painless, and sometimes heavy enough to fill the toilet with bright red blood and clots.2PubMed Central. Management of Diverticular Bleeding: Evaluation, Stabilization, Intervention, and Recurrence of Bleeding and Indications for Resection after Control of Bleeding One published case described a man in his forties admitted to the hospital after multiple episodes of large-volume painless rectal bleeding with clots, later found to have a diverticulum with a wide blood vessel at its base.3PubMed Central. Diverticular Bleeding: A Clinical Image The good news is that diverticular bleeding stops on its own in the majority of cases, though it can recur.
Angiodysplasia, which refers to clusters of fragile, abnormally dilated blood vessels in the colon wall, is another common cause, especially in older adults. These lesions tend to develop in the right side of the colon and are thought to result from degenerative changes with aging.4JAMA Internal Medicine. Angiodysplasia and Lower Gastrointestinal Tract Bleeding in Elderly Patients When they bleed, the pattern is similar to diverticular bleeding: painless and sometimes significant.
When Blood Clots Could Mean Something More Serious
While the common causes above account for most episodes, passing blood clots can also be a symptom of conditions that need prompt medical attention. Colorectal cancer and large polyps can bleed, sometimes intermittently and sometimes with enough volume to produce visible clots. Bleeding, along with bowel obstruction and perforation, is among the recognized acute complications of colon cancer.5PubMed Central. Diagnosis and management of acute complications in patients with colon cancer: bleeding, obstruction, and perforation Cancer-related bleeding does not have a single telltale presentation that distinguishes it from, say, diverticular bleeding by appearance alone, which is why doctors recommend investigating rectal bleeding rather than assuming it is harmless.
Ischemic colitis occurs when blood flow to part of the colon is temporarily reduced, damaging the tissue. It typically causes crampy abdominal pain along with bloody diarrhea and can produce clots. Most clinicians associate it with older patients who have cardiovascular problems, but it can also strike younger people with different risk factors, which sometimes makes the diagnosis tricky. About one in five patients with ischemic colitis ends up needing surgery, and those cases carry a high risk of complications.6PubMed Central. Ischemic colitis
Inflammatory bowel disease, including Crohn’s disease and ulcerative colitis, is another category. These conditions involve chronic inflammation of the gut lining and frequently cause bloody stools during flare-ups. The presence of ongoing diarrhea, abdominal cramping, weight loss, or fever alongside the bleeding generally points clinicians toward IBD rather than a structural cause like a diverticulum or hemorrhoid. One clinical teaching point is that when painless bleeding occurs without those systemic symptoms, IBD and ischemic colitis become less likely suspects.7PubMed Central. Acute Lower Gastrointestinal Bleeding
How Age Changes the Picture
Your age shifts the probability of different causes substantially. In older adults, diverticular disease and angiodysplasia dominate. Diverticula become increasingly common after age 40, and angiodysplasia is considered one of the most frequent causes of major lower intestinal bleeding in the elderly.4JAMA Internal Medicine. Angiodysplasia and Lower Gastrointestinal Tract Bleeding in Elderly Patients The risk of colorectal cancer also rises with age, which is why screening colonoscopies are recommended starting in your mid-forties.
In children and young adults, the menu of likely causes is different. A Meckel’s diverticulum, which is a small pouch left over from embryonic development, is a classic cause of painless rectal bleeding in children. It has traditionally been taught as presenting with painless bleeding, though a review of the literature found that young patients with Meckel’s diverticulum actually present more often with signs of intestinal obstruction than with bleeding alone.8PubMed 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 In younger adults, hemorrhoids and inflammatory bowel disease are more common sources of rectal bleeding than the vascular or structural problems that predominate in older patients.
Blood Thinners and Medications That Make It Worse
If you are taking a blood thinner or antiplatelet medication, even minor bleeding sources can produce impressively bloody bowel movements with clots. Drugs like warfarin, direct oral anticoagulants (apixaban, rivarelbans, and similar), and aspirin all interfere with your blood’s ability to clot. That means a hemorrhoid or small polyp that might have oozed slightly in someone not on these medications can bleed more freely and form visible clots.
Managing this situation is genuinely tricky for doctors, because stopping the blood thinner risks stroke or heart attack, while continuing it risks ongoing or worsening bleeding. A study examining how hospitals handle blood thinners in elderly patients admitted with rectal bleeding found wide variation in practice. Among patients on direct oral anticoagulants, only about 39% were restarted on the same medication at discharge. Roughly a quarter were switched to a different type, another quarter stopped anticoagulation entirely, and the rest had their restart delayed by a week or more. Patients on warfarin were more often restarted, while about a fifth stopped treatment. Among aspirin users, about 60% were restarted.9PubMed Central. Management of blood thinning medications in elderly populations presenting with rectal bleeding: Are we doing right? The takeaway for you: if you are on blood thinners and start passing blood clots, contact your doctor before stopping or changing your medication on your own. The decision involves weighing bleeding risk against the reason you are on the medication in the first place.
NSAIDs like ibuprofen and naproxen also deserve mention. They can cause bleeding anywhere in the GI tract, from the stomach down, and regular use increases the risk of both upper and lower gastrointestinal bleeding.
When You Should Go to the Emergency Room
Not every episode of bloody stool requires a trip to the ER, but some do. The warning signs that suggest you need urgent evaluation include: feeling lightheaded, dizzy, or faint; a rapid heartbeat; passing large amounts of blood, especially if it continues over hours; blood that is dark or maroon rather than bright red (suggesting it originated higher in the GI tract and spent more time in transit); significant abdominal pain; or any bleeding in someone who is already on blood thinners or has a known bleeding disorder.
Emergency departments have scoring systems to help decide which patients with lower GI bleeding can safely go home and which need to be admitted. The Oakland Score is one of the more widely studied. A systematic review and meta-analysis of risk scores found that the Oakland Score performed well for predicting both safe discharge and major bleeding, with high accuracy across multiple studies.10JAMA Network Open. Comparison of Risk Scores for Lower Gastrointestinal Bleeding: A Systematic Review and Meta-analysis A separate large study confirmed that at the lowest score threshold, the Oakland Score caught about 97% of patients who went on to have a bad outcome, meaning very few dangerous cases slipped through the net.11PubMed Central. Oakland score to identify low-risk patients with lower gastrointestinal bleeding performs well among emergency department patients That said, no scoring tool is perfect, and at least one single-center study found that the Oakland Score did not reliably predict safe discharge when used in isolation, particularly among patients who scored higher but were still sent home without complications.12PubMed Central. Single center assessment of the role of Oakland score among patients admitted for acute lower gastrointestinal bleeding In practice, doctors use these scores as one input alongside their clinical judgment, not as the final word.
If you are younger, have no other symptoms, pass a small amount of bright red blood once or twice, and feel otherwise fine, it is reasonable to call your primary care doctor and schedule an appointment rather than rushing to the ER. But if you have any of the red flags above, or if the bleeding is new and you are over 45 with no recent colonoscopy, err on the side of getting seen quickly.
How Doctors Track Down the Source
Colonoscopy is the workhorse investigation for lower GI bleeding. It lets a doctor visualize the entire colon, identify the bleeding source, and often treat it during the same procedure, whether that means clipping a bleeding vessel, cauterizing an angiodysplasia, or removing a polyp. For patients with active, heavy bleeding, CT angiography is sometimes done first to pinpoint the general location before a more targeted procedure.
There is an interesting practical finding about timing. A study comparing colonoscopy and catheter angiography after CT localization of bleeding found that the time from CT scan to procedure mattered more than which procedure was chosen. Catheter angiography happened faster on average, at about five hours compared to roughly fifteen and a half hours for colonoscopy, and shorter time to procedure was the only factor that significantly predicted whether doctors could confirm and treat the bleeding source.13PubMed. Colonoscopy Versus Catheter Angiography for Lower Gastrointestinal Bleeding After Localization on CT Angiography Speed matters because GI bleeding is intermittent; by the time the procedure happens, the bleeding may have stopped temporarily, making it harder to find the source.
Bleeding After a Colonoscopy or Polyp Removal
This is a scenario that catches people off guard. If you recently had a colonoscopy with polyp removal and now you are passing blood clots, you may be experiencing delayed post-polypectomy bleeding. When a polyp is removed, it leaves a small wound on the colon wall. A temporary scab forms over that wound, but sometimes the scab falls off before the tissue underneath has fully healed, exposing a blood vessel.14PubMed Central. Delayed Bleeding After Endoscopic Resection of Colorectal Polyps: Identifying High-Risk Patients Passage of stool over the site can dislodge the scab, which is why the bleeding sometimes coincides with a bowel movement.
Delayed bleeding after polypectomy happens in up to about 2% of patients and typically develops five to seven days after the procedure, though it can occur as late as 30 days afterward. Larger polyps carry a higher risk, roughly six times the rate seen with smaller ones. Older age, high blood pressure, and polyps located in the right side of the colon are additional risk factors.15Clinical Endoscopy. How Do I Manage Post-Polypectomy Bleeding? One analysis of delayed bleeding cases found that most episodes occurred on the first day after the procedure, with a small number stretching to day four or five.16Annals of Coloproctology. Analysis of Delayed Postpolypectomy Bleeding in a Colorectal Clinic If you have recently had polyps removed and start passing clots, call the doctor who performed the procedure. Most cases resolve on their own or with a repeat endoscopy, but the situation warrants prompt evaluation.
Bleeding After Pelvic Radiation
People who have received radiation therapy for cancers in the pelvic area, such as prostate, rectal, cervical, or bladder cancer, can develop a condition called radiation proctopathy. The radiation damages the blood vessels lining the rectum, leaving them fragile and prone to bleeding. Rectal bleeding is the most commonly reported symptom, and it can appear months or even years after treatment ends. Most cases are mild and self-limiting. When the bleeding is significant enough to cause anemia, first-line treatments include stool softeners to reduce straining and various medications applied directly to the rectum.17PubMed Central. Management of radiation-induced rectal bleeding If you have a history of pelvic radiation, mention it to any doctor evaluating your rectal bleeding, because it substantially changes the likely diagnosis.
When It Is Not Actually Blood
Before assuming the worst, consider whether what you are seeing is actually blood. Several foods and medications can turn stool red or dark in ways that convincingly mimic blood. Beets are the classic example, producing a vivid red discoloration that has sent many people into a panic. Red food dye in processed foods, candies, and breakfast cereals can do the same. A well-known case report from the 1970s described children presenting with alarming red stools that turned out to be caused by the dye in a newly released breakfast cereal. The authors noted that certain foods and drugs have long been known to alter stool color, and that physicians should be aware of these mimics to avoid unnecessary alarm.18Pediatrics. Benign Red Pigmentation of Stool Resulting from Food Coloring in a New Breakfast Cereal (The Franken Berry Stool)
Iron supplements and bismuth subsalicylate (the active ingredient in Pepto-Bismol) turn stool black, which can be mistaken for melena, the tarry black stool caused by digested blood from the upper GI tract. If you recently ate beets, drank a red sports drink, or started a new supplement, that could be the explanation. A simple way to check is to think about whether the color appeared abruptly after eating something specific and whether it goes away within a day or two. True blood in stool tends to recur, has a distinctive smell when it involves upper GI bleeding, and often comes with other symptoms like changes in bowel habits or abdominal discomfort.
What a Single Episode Versus Recurring Bleeding Tells You
A one-time episode of bright red blood after straining hard during a bowel movement, especially if you have a history of constipation or hemorrhoids, is the scenario that is least likely to represent something dangerous. The blood is usually on the surface of the stool or on the paper, and it stops within a day. Most clinicians would advise increasing fiber, staying hydrated, and following up if it happens again.
Recurring bleeding changes the equation. Even if each individual episode is small, repeated blood loss can add up to anemia over time. More importantly, intermittent bleeding is the pattern seen with polyps, early colorectal cancer, and angiodysplasia. These sources bleed, stop, and bleed again as conditions in the colon shift. If you notice blood in your stool on more than one or two occasions over a span of weeks, or if the pattern changes from occasional spotting to visible clots, you need a colonoscopy to identify the source. That recommendation holds regardless of age, though it becomes more pressing after 45, when the incidence of colorectal neoplasia starts climbing.
Dark or maroon-colored clots mixed into the stool, as opposed to bright red blood coating the outside, tend to suggest a source higher in the colon or even in the small intestine. Blood that has traveled farther through the GI tract has more time to be acted on by digestive enzymes, changing its color and consistency. This distinction is not perfectly reliable, since very brisk bleeding from higher up can still look bright red by the time it exits, but it gives your doctor a useful starting clue about where to look.
Infections and Food Poisoning
Bacterial infections of the colon can cause bloody diarrhea with clots, and in otherwise healthy people, this is one of the more common acute causes. Bacteria like Campylobacter, Shigella, certain strains of E. coli, and Salmonella can invade the lining of the colon, causing inflammation and ulceration that bleeds. The giveaway is usually the context: you also have diarrhea, cramping, fever, and the bleeding started alongside what clearly feels like a GI illness rather than appearing on its own. Infectious colitis usually resolves within a week with supportive care, though some bacterial causes require antibiotics. Stool cultures can identify the organism when the diagnosis is not obvious from the clinical picture.
Parasitic infections are a less common but real cause in certain settings, particularly after international travel. Entamoeba histolytica, the parasite behind amoebic dysentery, is the classic culprit and can produce bloody, mucousy stool. If you recently traveled to a region where this is endemic and develop bloody bowel movements, mention the travel history to your doctor, as the diagnostic and treatment pathway is different from standard bacterial food poisoning.