Gastrointestinal bleeding happens when damage to the lining of the digestive tract exposes underlying blood vessels. The cause depends largely on where the bleeding occurs. Peptic ulcers are the single most common source, accounting for roughly a third of all upper GI bleeds, while diverticular disease leads the list for lower GI bleeds at 20 to 50% of cases.
Upper vs. Lower GI Bleeding
The digestive tract is divided into upper and lower sections at a small ligament that anchors the end of the duodenum (the first part of the small intestine). Bleeding above that point typically shows up as vomiting blood or producing dark, tarry stools called melena. Bleeding below that point usually appears as bright red or maroon blood in the stool, known as hematochezia. These visual differences exist because blood that travels a longer distance through the gut gets partially digested, turning dark.
This distinction matters because the causes of upper and lower bleeding are quite different, and the approach to finding the source changes accordingly.
Peptic Ulcers and Stomach Inflammation
Peptic ulcers remain the top cause of upper GI bleeding. These are open sores that form in the stomach lining or the first stretch of the small intestine. When an ulcer erodes deep enough to reach a blood vessel, the result can range from a slow, invisible ooze to a sudden, heavy bleed.
The bacterium H. pylori is behind many of these ulcers. Virulent strains trigger a powerful inflammatory response, flooding the area with immune cells that release tissue-damaging enzymes and reactive oxygen molecules. At the same time, the infection reduces the gut’s ability to secrete bicarbonate, which normally neutralizes stomach acid in the duodenum. The combination of heightened inflammation and unchecked acid eats away at the protective mucosal lining.
Esophagitis (inflammation of the esophagus, often from acid reflux) accounts for about 24% of upper GI bleeds, while gastritis and duodenitis together make up another 30% or so. These conditions erode the surface lining in much the same way, just at different locations along the tract.
Medications That Damage the Gut Lining
NSAIDs like ibuprofen, aspirin, and naproxen are a major contributor to GI bleeding. These drugs work by blocking an enzyme that produces compounds involved in pain and inflammation. The problem is that the same enzyme also produces compounds that maintain the gut’s protective mucus barrier. When that barrier thins, stomach acid can reach and damage the tissue underneath, eventually opening a path to blood vessels.
Blood-thinning medications, including anticoagulants and antiplatelet drugs, don’t directly damage the gut lining. Instead, they make any existing source of bleeding harder for the body to seal off. A tiny erosion that might normally clot on its own can become a persistent or worsening bleed. The risk compounds when blood thinners are taken alongside NSAIDs.
Esophageal Varices and Liver Disease
Liver scarring (cirrhosis) creates a dangerous plumbing problem. As scar tissue replaces healthy liver cells, blood flow through the liver meets increasing resistance. Pressure builds in the portal vein, the major vessel carrying blood from the intestines to the liver. When that pressure climbs high enough, blood reroutes through smaller, thinner-walled veins in the esophagus and stomach that were never designed to handle it.
These swollen veins, called varices, begin to form when portal pressure reaches about 10 mmHg (normal is 1 to 5 mmHg). The risk of rupture and active bleeding rises significantly once pressure hits 12 mmHg or higher. A variceal bleed can be sudden and severe. Varices account for about 11% of upper GI bleeds overall, but among people under 60 with liver disease, variceal bleeding is by far the most common source, occurring in roughly 65% of cases in that group.
Mallory-Weiss tears, which are small rips in the lining where the esophagus meets the stomach, cause another 5 to 15% of upper GI bleeds. These tears often result from forceful or prolonged vomiting, which is one reason they’re seen more frequently in people with alcohol use disorders.
Diverticular Disease
Diverticulosis, the formation of small pouches in the colon wall, is the leading cause of lower GI bleeding. These pouches develop at weak spots where blood vessels penetrate the muscle layer of the colon. Over time, a blood vessel running along the dome of a pouch can erode and rupture, causing a sudden, painless gush of bright red or maroon blood. Most diverticular bleeds stop on their own, but they can recur.
This condition becomes increasingly common with age. Most people over 60 have some degree of diverticulosis, though only a fraction ever experience bleeding from it.
Hemorrhoids and Anal Fissures
Hemorrhoids are one of the most frequent sources of rectal bleeding and can account for up to 22% of lower GI bleeds in some studies. The blood is typically bright red, appears on toilet paper or in the bowl, and is often painless. Anal fissures, small tears in the lining of the anal canal, produce similar-looking bleeding but usually come with sharp pain during bowel movements. Neither condition is dangerous in most cases, but they can mask more serious causes of bleeding if assumed to be the explanation without investigation.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease cause chronic inflammation that damages the intestinal lining. Bleeding is a hallmark symptom of ulcerative colitis especially, since it targets the colon and rectum. The blood loss can be obvious (mixed into stool) or hidden, showing up only as iron deficiency anemia over time. IBD accounts for roughly 4 to 10% of lower GI bleeds, though in younger patients it represents a proportionally larger share of cases.
Vascular Malformations
Abnormal clusters of blood vessels in the gut wall, most commonly called angiodysplasia, are another important source of lower GI bleeding. These fragile, dilated vessels tend to develop in the right side of the colon and are more common in older adults. They can cause intermittent, low-volume bleeding that’s easy to miss, or occasionally more significant episodes. Recent studies put their contribution to lower GI bleeds at 3 to 6%, though older research reported rates as high as 40%, likely reflecting differences in how thoroughly other causes were ruled out.
Cancer and Polyps
Tumors in the GI tract bleed because they develop abnormal blood vessel networks and can outgrow their own blood supply, causing areas of tissue death and ulceration. Colorectal cancer accounts for up to 26% of all lower GI bleeding cases. The bleeding is often slow and chronic rather than dramatic, which means the first sign may be fatigue, shortness of breath, or unexplained iron deficiency anemia rather than visible blood in the stool. A positive fecal occult blood test (the screening test that detects hidden blood) is one of the primary ways colorectal cancer gets caught early.
In the upper GI tract, cancers of the esophagus and stomach can also bleed through the same erosive process. Among older adults, malignancy-related bleeding is significantly more common. One study found that cancer caused upper GI bleeding in nearly 14% of patients 60 and older, compared to just 3% of younger patients.
How Age Shifts the Likely Cause
In younger adults, variceal bleeding from liver disease dominates upper GI bleed cases, often linked to alcohol-related cirrhosis or viral hepatitis. In people over 60, peptic ulcers and cancers become proportionally much more common. For lower GI bleeding, diverticular disease and angiodysplasia are heavily weighted toward older adults, while IBD tends to present earlier in life, often between the teens and 40s.
This age pattern is one reason that new rectal bleeding in someone over 50 raises more concern and typically prompts a colonoscopy, while the same symptom in a 25-year-old is more likely to be hemorrhoids or IBD.
How the Source Gets Identified
For upper GI bleeding, an endoscopy (a thin, flexible camera passed through the mouth) is the primary diagnostic tool. For lower GI bleeding, colonoscopy after bowel preparation identifies the source in 69 to 90% of cases. When bleeding is coming from the upper tract, endoscopy finds it in only about 40% of cases, partly because blood can obscure the view during an active bleed.
When these standard scopes don’t find the source, imaging scans that detect blood flow can pick up active bleeding at rates as low as 0.5 mL per minute, and nuclear medicine scans can detect even slower bleeds at 0.1 mL per minute. Some GI bleeds are intermittent or very slow, making them notoriously difficult to pin down. These cases, called occult GI bleeding, sometimes require repeated testing or capsule endoscopy (a swallowable camera that photographs the small intestine) to find the culprit.