Gastrointestinal (GI) bleeds happen when damage to the lining of the digestive tract exposes underlying blood vessels. The single most common cause is peptic ulcer disease, responsible for 27% to 40% of all upper GI bleeding episodes. But the full list of causes spans the entire digestive system, from the esophagus to the rectum, and ranges from medication side effects to chronic inflammatory conditions.
GI bleeding is a major medical emergency, with an overall mortality rate of 2% to 10%. Upper GI bleeds tend to be more dangerous than lower GI bleeds, and variceal bleeding (from swollen veins in the esophagus) carries the highest risk, with up to 20% of those patients dying.
Upper vs. Lower GI Bleeds
The digestive tract is divided at a small muscular valve called the ligament of Treitz, which sits where the stomach empties into the small intestine. Bleeding above this point is classified as an upper GI bleed; bleeding below it is a lower GI bleed. The distinction matters because the causes, symptoms, and severity differ significantly between the two.
Upper GI bleeds are more common and generally more serious. Lower GI bleeds, while still potentially dangerous, stop on their own more often. The small bowel accounts for only about 5% of all lower GI bleeding sites, but it’s the most common source of “obscure” bleeding, the kind that’s hard to locate with standard testing.
Peptic Ulcers: The Leading Cause
Peptic ulcers are open sores that develop in the lining of the stomach or the first part of the small intestine. When an ulcer erodes deeply enough to reach a blood vessel, it bleeds. In studies involving more than 10,000 patients, peptic ulcer disease was the cause in 27% to 40% of upper GI bleeding cases.
Two things cause the vast majority of peptic ulcers. The bacterium H. pylori is responsible for nearly all duodenal ulcers and about 80% of gastric ulcers. Among patients who show up with a bleeding duodenal ulcer, roughly 71% test positive for H. pylori. For bleeding gastric ulcers, that number is between 61% and 79%. The use of nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen accounts for nearly all the remaining cases.
These two risk factors can also overlap. Someone with an undetected H. pylori infection who regularly takes NSAIDs for pain has a compounding risk, because both weaken the protective mucus layer that shields the stomach lining from its own acid.
Medications That Increase Risk
NSAIDs deserve special attention because they’re so widely used. These drugs work by blocking enzymes involved in inflammation, but those same enzymes help maintain the stomach’s protective lining. Without that protection, acid eats into the tissue.
Low-dose aspirin, even at the doses commonly taken for heart protection, increases the risk of upper GI bleeding by 1.6 to 4 times compared to not taking it. The risk climbs further when aspirin or NSAIDs are combined with blood thinners like warfarin or newer anticoagulants. Corticosteroids taken alongside NSAIDs also amplify the danger.
If you take any of these medications regularly, the risk doesn’t mean you should stop them on your own. It does mean the combination matters, and your prescriber should know everything you’re taking, including over-the-counter painkillers.
Esophageal Varices and Liver Disease
Varices are swollen, fragile veins that develop in the esophagus or upper stomach when blood flow through the liver is blocked. This is called portal hypertension, and it’s most commonly caused by cirrhosis from chronic alcohol use or hepatitis. Two factors drive it: increased resistance inside the scarred liver and increased blood flow in the vessels feeding the liver. Together, they force blood to reroute through smaller veins that weren’t designed to handle the pressure.
When these distended veins rupture, the bleeding can be massive and life-threatening. Variceal bleeding carries the highest mortality of any GI bleed type, with up to 20% of patients dying. This is why people with known liver disease are often screened for varices before they ever bleed.
Diverticular Bleeding
Diverticulosis, the presence of small pouches that bulge outward through the colon wall, is the leading cause of significant lower GI bleeding. It accounts for 25% to 60% of cases where blood loss is severe enough to affect blood pressure or require transfusion.
About 20% of people with diverticular disease will experience bleeding at some point. The good news is that it stops on its own in roughly 80% of cases. For about 5% of patients, though, the bleeding can be massive. One counterintuitive detail: although 75% of diverticula form on the left side of the colon, right-sided diverticula are responsible for 50% to 90% of the bleeding. The blood vessels on the right side of the colon are more exposed as they drape over the dome of each pouch, making them more vulnerable to injury.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both cause chronic inflammation in the digestive tract, and both can lead to bleeding. The patterns differ. Up to 50% of people with ulcerative colitis experience mild to moderate rectal bleeding as a symptom of their disease, but massive, life-threatening hemorrhage is uncommon, occurring in about 4% of cases.
Crohn’s disease causes severe lower GI bleeding less frequently, affecting roughly 0.6% to 5.5% of patients over the course of their illness. When it does happen, it tends to recur. Even after the initial bleeding is controlled, 19% to 50% of Crohn’s patients with a severe bleed will experience it again. The mortality rate for these severe episodes ranges from 0% to 14.3%, depending on the severity and how quickly it’s managed.
Vascular Malformations
Angiodysplasias are clusters of fragile, abnormally formed blood vessels in the wall of the digestive tract. They’re a major cause of GI bleeding in older adults, particularly in the small bowel and right colon. These lesions can bleed intermittently, making them notoriously hard to diagnose because they may not be actively bleeding during testing.
Several conditions raise the risk: chronic kidney failure, aortic stenosis (a narrowing of a heart valve), chronic lung disease, and the use of blood thinners like warfarin. Women are affected more often than men. Because angiodysplasias tend to cause slow, chronic blood loss rather than a sudden hemorrhage, the first sign may be unexplained anemia rather than visible bleeding.
Colorectal Cancer
Tumors in the colon or rectum can bleed as they grow, ulcerate, or erode into surrounding tissue. This bleeding is often slow and invisible to the naked eye, which is why stool-based screening tests work: they detect tiny amounts of hidden blood. Massive bleeding from colorectal cancer is less common, occurring in 5% to 20% of cases in different studies, but any new rectal bleeding in someone over 45, or anyone with a family history of colon cancer, warrants investigation.
How Symptoms Point to the Source
The appearance of the blood offers clues about where in the digestive tract the bleeding is coming from. Vomiting red blood (hematemesis) points to an active upper GI bleed, typically from a peptic ulcer, a vascular lesion, or a ruptured varix. Vomiting material that looks like dark coffee grounds means upper GI bleeding that has slowed or stopped, because stomach acid converts red blood to a dark brown color.
Black, tarry stools (melena) typically signal upper GI bleeding, though the source can also be the small bowel or right colon. The dark color comes from blood being partially digested as it moves through the intestines. Bright red blood from the rectum (hematochezia) usually means a lower GI source, but it can also occur during a vigorous upper GI bleed when blood moves through the intestines quickly enough that it doesn’t have time to darken.
Not all GI bleeds produce visible symptoms. Slow, chronic bleeds may only show up as iron-deficiency anemia: fatigue, pale skin, shortness of breath with exertion, and lightheadedness. This is particularly common with angiodysplasias and small colorectal tumors.