Blood in Stool: Causes, Colors, and When to Worry

Seeing blood after a bowel movement is one of the sharpest jolts your gut can hand you. The first thing to know: most of the time it comes from something benign and treatable near the anus, not something dangerous. The second thing to know: it can also be the first sign of something you do not want to ignore, and there is no way to sort the two by feel alone.

The color of the blood is the most useful clue you can hand to a doctor. Bright red on the paper is different from bright red mixed through stool, which is different from maroon, which is different from black and tarry. Each of those maps to a rough location in the digestive tract and a shorter list of likely causes. Below is how the medical workup actually thinks about it, what the numbers say, and the specific combinations that flip this from a home problem to a same-day evaluation.

The First Question a GI Doctor Asks: What Color?

Blood changes color as it moves through the gut. Fresh blood from the anus or lower rectum barely has time to interact with digestive enzymes, so it stays bright red. Blood from higher up - the colon, small intestine, or stomach - gets partially or fully digested, which turns it maroon, dark brown, or black depending on how long the trip took.

Clinicians use three terms for this and they map cleanly onto what you would actually see:

• Hematochezia: bright red or maroon blood from the rectum. Almost always a lower GI source (colon, rectum, anus), though massive upper GI bleeding can occasionally look like this because the blood moves too fast to be digested.
• Melena: black, tarry, sticky, foul-smelling stool. Blood that has been degraded by stomach acid and gut bacteria. Almost always an upper GI source - the stomach, duodenum, or small intestine.
• Occult blood: blood present in stool but not visible to the naked eye, detected by a stool test (like a FIT). Can come from anywhere in the GI tract.

The NIH clinical review on hematemesis, melena, and hematochezia makes the anatomic distinction concrete: bleeding above the ligament of Treitz (roughly the junction of the duodenum and jejunum) shows up as vomited blood or melena, while bleeding below it usually presents as hematochezia. That single anatomic landmark is why color matters so much.

Bright Red Blood: The Anal and Rectal Causes

If the blood is bright red, especially if you see it only on toilet paper or dripping into the bowl, the source is almost always in the last few centimeters of the GI tract. Two conditions dominate this category.

Hemorrhoids

Hemorrhoids are swollen vascular cushions in the anal canal, and they are the single most common cause of visible rectal bleeding. Symptomatic internal hemorrhoids classically produce painless bright red bleeding at the end of defecation, sometimes with mucus discharge, itching, or a sensation of prolapse. External hemorrhoids are more likely to hurt and less likely to bleed heavily.

The scale: about 1 in 20 Americans has symptomatic hemorrhoids at any given time, and roughly half of adults over 50 will experience them, per the NIDDK. In hospitalized lower GI bleeding cases, hemorrhoids account for about 17% of identified sources (Strate and Gralnek, 2013). We covered the full picture of what hemorrhoids feel like, how they are graded, and what actually works to treat them in our hemorrhoids symptoms and treatment guide.

Anal fissures

An anal fissure is a small tear in the skin lining the anal canal, usually caused by passing a hard, large stool. The presentation is unmistakable to anyone who has had one: sharp, tearing pain during defecation that can last for hours afterward, plus bright red streaking on the toilet paper or a small amount of blood coating the stool.

Fissures are common. The population-based cohort by Mapel et al., 2014 put the overall annual incidence at about 1.1 per 1,000 person-years, peaking in adults 25 to 34. Chronic constipation is by far the biggest risk factor, present in about 14% of fissure patients versus 4% of controls. Most acute fissures heal within a few weeks with fiber, hydration, warm sitz baths, and topical treatments. Chronic fissures (unresolved past 6 to 8 weeks) sometimes need botulinum toxin or lateral internal sphincterotomy.

The pattern that fits both

Bleeding from hemorrhoids or fissures typically:

• Is bright red, not dark or maroon
• Appears on the paper, on the outside of the stool, or dripping into the bowl, not mixed through
• Is a small to moderate amount - streaks, spots, or a tablespoon or two at most
• Is associated with constipation, straining, or hard stools
• Stops within a day or two once bowel habits soften

Bleeding that fits this pattern in someone under 45 with no other symptoms is usually treated presumptively. But "usually" is not "always." A one-time bleed that resolves is different from recurring bleeding, and no primary care doctor will let recurring rectal bleeding go without at least a rectal exam.

Blood Mixed Through Stool: The Colonic Causes

When blood is mixed into the stool itself - not coating it, not just on the paper - the source is usually higher in the colon. This changes the differential list significantly.

Diverticular bleeding

Diverticula are small out-pouchings in the colon wall that form as people age. About 30% of adults over 50 have them; roughly half of adults over 60 do. Most are silent forever. But when a small artery in the neck of a diverticulum ruptures, the result is sudden, painless, sometimes large-volume bleeding - typically dark red or maroon, often with clots.

Diverticular bleeding is the single most common source of lower GI bleeding requiring hospitalization, accounting for about 47% of cases in national data (Strate and Gralnek, 2013). Most stops on its own, but recurrence is common, and heavy bleeds need urgent colonoscopy or interventional radiology. If you have known diverticulosis and start passing maroon stool, that is an ER trip, not a clinic call. The condition itself is separate from diverticulitis - inflammation of a diverticulum - which we broke down in our diverticulitis symptoms article.

Inflammatory bowel disease

Ulcerative colitis and Crohn's disease both cause visible bleeding, but the patterns differ.

Ulcerative colitis classically presents with frequent, urgent, small-volume bloody diarrhea containing mucus. The bleeding is usually bright red because it originates in the rectum and left colon, close to the anus. Nighttime symptoms that wake you from sleep, tenesmus (the feeling that you have to go without being able to), and unintentional weight loss are the features that separate UC from IBS.

Crohn's disease can affect any part of the GI tract, so bleeding is more variable - sometimes dark, sometimes bright, sometimes intermittent, sometimes absent even during active flares. Perianal disease (fissures, fistulas, abscesses) is a Crohn's-specific complication that can produce bleeding on its own.

IBD affects an estimated 2.4 to 3.1 million U.S. adults (Lewis et al., 2023), and diagnosis usually comes between ages 15 and 35 with a second smaller peak in the 50s and 60s. Fecal calprotectin, a stool inflammation marker, is one of the most useful non-invasive tests for separating IBD from IBS in the workup.

Colorectal polyps

Adenomatous polyps are precancerous growths in the colon lining. Most bleed intermittently and in small amounts, often producing occult (invisible) blood detected only by a FIT test rather than visible blood. But larger polyps or those in the left colon and rectum can produce visible bleeding, usually maroon or bright red mixed into stool. Polyps are the second most common identified source of lower GI bleeds requiring hospitalization, at about 20% (Strate and Gralnek, 2013). Screening colonoscopy exists specifically to find and remove these before they become cancer.

Colorectal cancer

Rectal bleeding is the most common presenting symptom of colorectal cancer. In symptomatic patients, blood in stool has one of the strongest predictive values of any single sign. A large 2025 analysis presented by the American College of Surgeons found that rectal bleeding raised the odds of a colorectal cancer diagnosis by roughly 8.5 fold in adults under 50 (ACS press release, 2025). About 45% of young-onset CRC patients reported rectal bleeding at diagnosis.

This matters more now than it used to. Colorectal cancer incidence in adults under 55 has been rising by roughly 2% per year since the mid-1990s, and CRC is now the leading cause of cancer death in men under 50 and the second in women under 50 (American Cancer Society, CA: A Cancer Journal for Clinicians, 2024). The USPSTF and NIDDK now recommend average-risk screening start at age 45, down from 50. The full picture on early symptoms, family history, and screening options is in our colorectal cancer warning signs guide.

Black, Tarry Stool: The Upper GI Emergency

Melena is unmistakable when you see it: black, sticky, tar-like stool with a distinctive foul odor. It means blood has been in the GI tract long enough for stomach acid and bacteria to break down hemoglobin into hematin, which is what gives it the color and texture. As a rule, it takes at least 50 to 100 mL of blood to produce a single melenic stool.

Most melena comes from the upper GI tract - stomach, duodenum, or upper small intestine. The most common causes:

• Peptic ulcer disease - by far the most common cause of upper GI bleeding, usually related to H. pylori infection or chronic NSAID use (ibuprofen, naproxen, aspirin)
• Erosive gastritis or esophagitis - often related to alcohol, NSAIDs, or severe GERD
• Esophageal or gastric varices - enlarged veins from portal hypertension in cirrhotic patients, and these can bleed catastrophically
• Mallory-Weiss tear - a tear at the junction of the esophagus and stomach after severe vomiting or retching

One important caveat: bismuth subsalicylate (Pepto-Bismol) and iron supplements can turn stool black without any bleeding at all. Bismuth-related black stool is usually more grey-black than tarry and lacks the distinctive smell of true melena. If you have started either in the last week, that is worth mentioning before assuming the worst - but not worth assuming as the cause on your own if you feel unwell or lightheaded.

Melena that is not clearly medication-related is a same-day evaluation, and heavy melena with lightheadedness, fast heart rate, or fainting is an emergency room trip.

Less Common but Worth Knowing

Angiodysplasia

Angiodysplasia is an abnormal, fragile blood vessel in the wall of the colon or small bowel that can bleed painlessly and intermittently. It is a common cause of chronic occult GI blood loss and iron deficiency anemia in adults over 60. Diagnosis is usually made at colonoscopy or capsule endoscopy.

Ischemic colitis

Ischemic colitis happens when blood flow to a segment of the colon is temporarily reduced, causing the mucosa to slough. The classic presentation is sudden crampy left-sided abdominal pain followed within 24 hours by bright red or maroon rectal bleeding. It is more common in older adults with vascular disease, and it usually resolves with supportive care but occasionally needs surgery.

Infectious colitis

Certain bacterial infections - Shigella, Salmonella, Campylobacter, invasive E. coli (including O157:H7), and Clostridioides difficile - can inflame the colon lining enough to produce bloody diarrhea. These usually have an acute onset with fever, cramping, and a clear exposure (food poisoning, travel, recent antibiotics). C. difficile in particular is worth a stool test if you have had antibiotics in the last three months and develop new mucus-y or bloody diarrhea.

Radiation proctitis

Adults who have had pelvic radiation for prostate, cervical, or rectal cancer sometimes develop chronic rectal bleeding months to years later from radiation damage to rectal blood vessels. This is treatable but usually needs endoscopic evaluation to distinguish from recurrent cancer.

Solitary rectal ulcer syndrome

Chronic straining and internal rectal prolapse can create an ulcer in the rectum that leaks blood and mucus. It is uncommon but frequently missed. If you have strained hard for years managing chronic constipation and are now bleeding, this belongs on the list.

What Color Blood, What Location: A Quick Map

None of this is perfectly predictive - a brisk upper GI bleed can look bright red because it moves fast, and a slow right colon bleed can look dark because it sits. But as a starting point:

• Bright red on paper only, or dripping into the bowl: hemorrhoids, anal fissure
• Bright red coating a formed stool: hemorrhoids, fissure, low rectal source
• Bright red or maroon mixed throughout stool, with clots: diverticular bleed, IBD flare, polyp, cancer, ischemic colitis, angiodysplasia
• Bloody diarrhea: IBD flare, infectious colitis, ischemic colitis
• Black, tarry, sticky (melena): upper GI bleed - ulcer, gastritis, varices - or right colon bleed with slow transit
• No visible blood but positive FIT: occult bleeding anywhere in the tract, workup driven by age and risk

The Red Flags That Change the Timeline

Any visible blood in stool is worth a doctor's opinion. Some combinations turn it from "make an appointment" to "go today" or "go to the ER":

• Large volume bleeding - the toilet bowl looks obviously bloody, blood on more than one bowel movement, or passing clots
• Lightheadedness, fainting, fast heart rate, cold sweats - signs of significant blood loss and hemodynamic compromise
• Black tarry stool (melena) not clearly explained by bismuth or iron
• Severe abdominal pain with bleeding
• Fever with bleeding
• Unintentional weight loss of more than 5% of body weight
• Change in stool caliber (pencil-thin, ribbon-like) that persists more than a couple of weeks
• Persistent change in bowel habits lasting more than 4 weeks
• Age over 45 with any new rectal bleeding
• Family history of colorectal cancer, polyps, or IBD, at any age
• Anticoagulant or antiplatelet use (warfarin, apixaban, rivaroxaban, dabigatran, clopidogrel) with new bleeding

If you are on a blood thinner, the threshold for calling your prescriber is low. Bleeding on anticoagulation can escalate quickly and often needs urgent labs and dose adjustment even when the source is benign. The American Family Physician 2020 review on acute lower GI bleeding makes anticoagulant assessment a routine part of the initial evaluation.

What the Workup Looks Like

The evaluation follows the color and the story. In broad strokes:

• Digital rectal exam - the first step at almost every visit for rectal bleeding, and one of the most useful. It confirms external hemorrhoids, most fissures, and low rectal masses that would otherwise be missed.
• Anoscopy - a short scope of the anal canal, done in the office, to look at internal hemorrhoids and low rectal lesions.
• Colonoscopy - the gold standard for anyone with unexplained visible bleeding, blood mixed through stool, or bleeding over age 45. Can biopsy suspicious lesions and treat many bleeding sources at the same time.
• Upper endoscopy (EGD) - for melena or suspected upper GI source.
• Stool studies - fecal calprotectin (for IBD versus IBS), infectious workup (C. difficile toxin, culture, PCR panels), and FIT for occult blood screening.
• Labs - CBC to look for anemia, iron studies for chronic slow bleeds, coagulation panel especially if on blood thinners.

If your primary care doctor sees you and the story is a young adult with bright red bleeding, constipation, and no red flags, the reasonable first move is often to treat the presumed hemorrhoid or fissure with fiber, hydration, sitz baths, and topical care, and reassess in 4 to 6 weeks. If bleeding continues, that becomes gastroenterology's problem to sort. For a broader sense of when a GI referral is the right call, we walked through that in our when to see a gastroenterologist piece.

What You Can Do Now

If the bleeding is small, bright red, and clearly tied to a hard stool, and you feel otherwise well:

• Soften the stool. Aim for 25 to 38 grams of fiber a day, spread out and increased gradually to avoid gas. Psyllium (Metamucil) is the best-studied single intervention. Details in our fiber guide.
• Drink water. 2 to 3 liters of fluid daily. Fiber without water is a recipe for worse constipation, not better.
• Sit less, elevate feet on a footstool. A footstool that flexes the hips above 90 degrees straightens the anorectal angle and reduces straining - one of the cheapest evidence-based interventions in gastroenterology.
• Stop straining. If nothing is happening in 10 minutes, get up and try later. Prolonged pressure worsens both hemorrhoids and fissures.
• Sitz baths. 10 to 15 minutes in warm water two or three times a day for the first week helps both fissures and inflamed hemorrhoids.

If bleeding continues past a week or two despite these changes, or if any red flag above appears, that is the doctor visit, not a wait-and-see.

The Case for Writing It Down

Rectal bleeding is one of the symptoms doctors get the least useful description of, because most people would rather not think about it in detail. A short log - a week or two of when the bleeding happened, what color it was, whether it was on the paper or in the bowl or mixed through stool, how much, what the stool itself looked like on the Bristol scale, and what you ate - takes the guesswork out of the first clinic visit.

That kind of log also catches the pattern that a single bathroom trip can hide: bleeding that seemed to stop after one episode but is actually recurring weekly, or bleeding that clusters with certain foods, or stools that are shifting toward looser and looser without you noticing. If you are not sure whether stool form itself is changing, our Bristol Stool Chart guide walks through what the seven types look like and what they signal. Number Two logs form, color, blood, pain, and meals in a few taps per bowel movement, which is most of what a gastroenterologist wants to see when a symptom like this walks in the door.