A bleeding ulcer happens when an open sore in the stomach or upper small intestine erodes deep enough to damage a blood vessel. The two most common causes are infection with the bacterium H. pylori and regular use of nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and aspirin. Less common triggers include rare hormone-producing tumors and severe physiological stress from critical illness. Understanding what drives this damage helps explain why some ulcers stay manageable while others become dangerous.
How H. pylori Damages the Stomach Lining
Your stomach is lined with a thick layer of mucus and a coating of fatty molecules that shield the tissue beneath from its own acid. H. pylori, a spiral-shaped bacterium that colonizes the stomach, produces enzymes that break down both of these barriers. It releases a protease that degrades the mucus and a lipase that disrupts the protective fatty layer on the surface of stomach cells. Once those defenses are stripped away, acid diffuses back into the exposed tissue and starts to erode it.
The bacterium also produces large amounts of an enzyme called urease, which generates ammonia. That ammonia is directly toxic to the cells lining the stomach. Over time, the combination of acid exposure and chemical damage creates an ulcer. If the erosion reaches a blood vessel in the stomach wall, bleeding begins. H. pylori infection is extremely common worldwide, but only a fraction of infected people develop ulcers. Factors like the specific bacterial strain, individual immune response, and additional irritants like smoking or alcohol influence who progresses from infection to ulcer to bleeding.
NSAIDs and the Loss of Protective Prostaglandins
NSAIDs work by blocking enzymes called COX-1 and COX-2, which produce prostaglandins. In joints and muscles, fewer prostaglandins means less inflammation and pain. In the stomach, prostaglandins do something entirely different: they stimulate the secretion of bicarbonate (which neutralizes acid near the stomach wall), maintain blood flow to the lining, and help damaged tissue heal. When you take an NSAID, you suppress all of those protective functions at once.
COX-1 is the enzyme primarily responsible for maintaining the bicarbonate barrier and mucosal blood flow. COX-2 plays a key role in healing ulcers that have already formed. Standard NSAIDs like ibuprofen, naproxen, and aspirin inhibit both, which is why they can cause new ulcers and prevent existing ones from repairing themselves. The risk climbs with higher doses, longer use, and older age. Even low-dose aspirin taken daily for heart protection carries a measurable ulcer risk.
Drug Combinations That Multiply the Risk
Taking NSAIDs alongside certain other medications dramatically increases the chance of upper gastrointestinal bleeding. A large study published in Gastroenterology found that combining a standard NSAID with a corticosteroid (like prednisone) raised the bleeding risk nearly 13 times compared to using neither drug. Combining an NSAID with an SSRI antidepressant also added meaningful extra risk beyond what either drug would cause alone. The SSRI connection exists because these antidepressants reduce serotonin in platelets, making it harder for blood to clot at an ulcer site. If you take any of these combinations regularly, the cumulative effect on your stomach lining is substantially greater than any single drug on its own.
Rare Causes: Gastrinomas and Excess Acid
Zollinger-Ellison syndrome is an uncommon condition in which small tumors called gastrinomas develop in the pancreas or upper small intestine. These tumors secrete large amounts of the hormone gastrin, which signals the stomach to produce acid. The result is a stomach churning out far more acid than normal, overwhelming the mucus barrier and causing ulcers that are difficult to treat and prone to bleeding. People with this syndrome often develop multiple ulcers or ulcers in unusual locations, such as the lower part of the small intestine, and may also experience chronic diarrhea from the acid flooding into the gut.
Stress Ulcers in Critical Illness
The “stress” behind stress ulcers is not everyday psychological stress. It refers to the extreme physiological strain of critical illness: major burns, head injuries, sepsis, or organ failure requiring intensive care. The connection has been recognized for more than 150 years, when researchers first described duodenal ulcer bleeding in patients with extensive burn injuries. In critically ill patients, reduced blood flow to the stomach lining, combined with the body’s disrupted ability to maintain its normal protective barriers, allows acid to damage tissue rapidly. These ulcers can develop within hours of a major physiological insult.
How a Bleeding Ulcer Shows Up
The symptoms depend on how fast the ulcer is bleeding. A slow bleed may not produce dramatic signs at first. You might notice dark, tarry stools, which occur when blood from the upper digestive tract is broken down as it passes through the intestines. It takes roughly half a cup to a full cup of blood in the upper gut to produce this distinctive black stool, and it can continue looking that way for several days even after bleeding stops.
A faster bleed can cause vomiting of blood, which may appear bright red or look like dark brown, granular material resembling coffee grounds. The coffee-ground appearance happens when stomach acid converts the red pigment in blood to a brown compound. Rapid blood loss triggers signs of shock: a racing heart, pale and clammy skin, dizziness or fainting, confusion, and a drop in blood pressure when standing up. Losing more than two units of blood often causes measurable drops in blood pressure along with a spike in heart rate.
What Happens During Diagnosis
Doctors evaluate a bleeding ulcer using an endoscopy, a thin camera passed through the mouth into the stomach. During this procedure, they classify what they see at the ulcer site, which directly predicts how likely the ulcer is to keep bleeding or start again. An ulcer that is actively spurting blood rebleeds about 55% of the time without treatment. One with a visible blood vessel that isn’t currently bleeding still rebleeds about 43% of the time. An ulcer with a clot stuck to its surface carries a 22% rebleeding risk, while a flat discolored spot drops to around 10%. A clean-based ulcer, where the floor of the sore looks white or pale without blood vessel remnants, rebleeds only about 5% of the time.
These distinctions matter because they determine how aggressively the ulcer needs to be treated during the endoscopy itself, and how closely you’ll be monitored afterward.
Why Some Ulcers Bleed and Others Don’t
An ulcer becomes a bleeding ulcer when the erosion is deep enough to reach a blood vessel. Several factors push an ordinary ulcer in that direction. Continued NSAID use prevents healing. Untreated H. pylori infection sustains the inflammatory damage. Alcohol irritates already-compromised tissue. Smoking reduces blood flow to the stomach lining and slows repair. In people who carry H. pylori and also take NSAIDs, the risks compound: a study of elderly NSAID users found that H. pylori infection nearly doubled the odds of developing a bleeding ulcer in that group, accounting for roughly one in four bleeding events.
The size and location of the ulcer also play a role. Ulcers on the back wall of the duodenum sit near a major artery, making them more likely to cause severe hemorrhage if they erode deeply enough. Larger ulcers have more surface area to encounter a blood vessel. And ulcers that have bled once are at higher risk of bleeding again, particularly if the underlying cause, whether infection or medication, hasn’t been addressed.