Gastrointestinal (GI) bleeding has two broad categories based on where it starts: upper bleeding (from the esophagus, stomach, or first part of the small intestine) and lower bleeding (from the colon and rectum). Peptic ulcers are the single most common cause overall, responsible for roughly a third of upper GI bleeds, while diverticular disease leads the list for lower GI bleeds at 20 to 50% of cases. The specific cause depends heavily on your age, medications, alcohol use, and underlying health conditions.
Upper GI Bleeding: The Main Culprits
The stomach and esophagus are where most serious GI bleeding originates. Peptic ulcers account for 32 to 36% of upper GI bleeds in recent studies, down from the roughly 50% seen in older data. These ulcers are open sores in the stomach lining or the upper small intestine, caused primarily by two things: a bacterial infection called H. pylori, and regular use of anti-inflammatory painkillers like ibuprofen or aspirin.
H. pylori causes damage through a surprisingly aggressive process. The bacteria burrow through the protective mucus layer of the stomach using whip-like tails, then attach directly to the stomach lining. Once there, they inject proteins into your cells that disrupt normal cell signaling and weaken the tight junctions holding the lining together. At the same time, the infection triggers a chronic inflammatory response, recruiting immune cells that release chemicals damaging the very tissue they’re trying to protect. Over time, this breaks down the stomach’s defenses and allows acid to eat into the tissue, forming ulcers that can erode into blood vessels.
After ulcers, the next most common causes of upper bleeding are inflammation of the esophagus (24% of cases), stomach inflammation (18 to 22%), inflammation of the upper small intestine (13%), and swollen veins in the esophagus called varices (11%). Mallory-Weiss tears, which are rips in the esophageal lining from forceful vomiting or retching, cause 5 to 15% of cases. Abnormal blood vessel clusters and tumors account for smaller shares.
Lower GI Bleeding: The Main Culprits
Diverticular bleeding is the leading cause of lower GI bleeding. Diverticula are small pouches that form in the colon wall, and when a blood vessel near one of these pouches erodes or ruptures, the result can be dramatic. About 10 to 15% of people with diverticular disease will experience bleeding, though severe hemorrhage happens in only 3 to 5%. The good news: 75 to 90% of diverticular bleeds stop on their own. This type of bleeding is rare before age 40 and becomes increasingly common with age.
Hemorrhoids are another major contributor. Bleeding from internal hemorrhoids is typically painless and shows up as bright red blood on toilet paper or in the bowl. Most of the time the amount is small, but a small subset of people experience significant or chronic blood loss that leads to anemia.
Abnormal blood vessel formations in the colon, most commonly found on the right side, are a third important source. These are acquired over a lifetime, so they become more common as you get older. They can bleed intermittently and are sometimes tricky to locate.
How NSAIDs Increase Bleeding Risk
Common over-the-counter painkillers carry real, measurable risks for GI bleeding. NSAIDs work by blocking enzymes that produce inflammation, but those same enzymes also help maintain the stomach’s protective lining. Without that protection, acid can damage the tissue directly.
Not all NSAIDs carry equal risk. A large meta-analysis found that ibuprofen roughly doubles the odds of GI bleeding (2.3 times baseline risk), making it the least risky among standard NSAIDs. Diclofenac raises risk about 3.4 times, naproxen about 4.3 times, and indomethacin about 5.5 times. Piroxicam carries one of the highest risks at roughly 9 times baseline. The risk climbs further when NSAIDs are combined with certain antidepressants (SSRIs). Overall, regular NSAID use can increase the risk of upper GI bleeding up to fourfold.
Selective COX-2 inhibitors like celecoxib were designed to be gentler on the stomach, and the data supports this: celecoxib showed no statistically significant increase in GI bleeding risk in pooled analyses.
How Alcohol Damages the GI Tract
Heavy alcohol use contributes to GI bleeding through multiple pathways. The most dangerous is liver cirrhosis, which restricts blood flow through the liver. When blood can’t flow through the liver efficiently, it backs up into smaller veins in the esophagus and stomach. These veins aren’t built to handle that pressure, so they swell into varices that can rupture and bleed heavily. Esophageal varices are among the most life-threatening causes of GI bleeding.
Alcohol also causes bleeding more directly. Chronic drinking irritates and inflames the stomach lining. And forceful vomiting, common during heavy drinking episodes, can physically tear the esophageal lining (Mallory-Weiss tears), which can produce significant blood loss.
Inflammatory Bowel Disease
In both ulcerative colitis and Crohn’s disease, the immune system sends white blood cells into the digestive tract lining, where they release inflammatory chemicals that damage tissue and cause bleeding. The pattern differs between the two conditions.
Ulcerative colitis typically causes bloody diarrhea, urgency, and a persistent feeling of needing to go even after a bowel movement. Because this condition is confined to the colon and starts at the rectum, cramping and bleeding tend to center in the lower abdomen. Crohn’s disease more often involves abdominal pain with non-bloody diarrhea and weight loss, particularly when the small intestine is affected, though bleeding can still occur.
How to Tell Where the Bleeding Is
The appearance of blood offers strong clues about its source. Black, tarry stools (called melena) strongly suggest upper GI bleeding. Blood from the stomach or esophagus turns dark and tar-like as it’s digested during its trip through the intestines. If you’re vomiting blood, the source is almost certainly in the upper tract.
Bright red blood passed with or in stools (hematochezia) usually points to a colonic source. There are exceptions in both directions, though. If intestinal transit is slow, bleeding from as far down as the first part of the colon can produce black stools. And very rapid arterial bleeding from a stomach ulcer can move through the system fast enough to come out bright red.
Age Makes a Difference
The likely cause of GI bleeding shifts substantially across the lifespan. In newborns, the most common causes include clotting problems from vitamin K deficiency, cow’s milk protein allergy, and stress-related stomach irritation. In infants, acid reflux and esophageal inflammation take the lead. Toddlers are prone to bleeding from caustic substance or foreign body ingestion alongside reflux and stomach inflammation. In children over three, Mallory-Weiss tears, ulcers, and esophageal varices enter the picture, along with bleeding from blood disorders like leukemia.
In older adults, the landscape changes again. Diverticular disease and abnormal blood vessel formations become dominant causes of lower GI bleeding, both increasing with age. Peptic ulcers remain important, and GI cancers become a growing concern. NSAID use, which is more common in older adults managing chronic pain or heart disease, adds further risk.
How Serious Is GI Bleeding?
GI bleeding ranges from trivial to fatal. A streak of blood on toilet paper from a hemorrhoid is rarely dangerous. A bleeding ulcer or ruptured varix can be a medical emergency. Between 2018 and 2023, over 133,000 deaths in the United States were linked to upper GI bleeding alone, yielding a national mortality rate of about 25.5 per 100,000 people. Black Americans face the highest mortality at 46.2 per 100,000, nearly twice the rate seen in white Americans. Geographic disparities are also striking, with mortality rates varying more than sevenfold across different states.
Signs that bleeding may be serious include dizziness or lightheadedness, rapid heartbeat, weakness, vomiting blood or material that looks like coffee grounds, and black tarry stools. Chronic, low-level bleeding is more subtle and often shows up as unexplained fatigue or anemia discovered on a blood test.