Gastrointestinal (GI) bleeding has dozens of possible causes, ranging from common conditions like ulcers and hemorrhoids to serious ones like liver disease and cancer. The cause depends largely on where the bleeding originates: the upper digestive tract (esophagus, stomach, and first part of the small intestine) or the lower tract (the rest of the small intestine, colon, and rectum). Peptic ulcers are the single most common cause of upper GI bleeding, while diverticular disease leads the list for lower GI bleeding.
Upper vs. Lower GI Bleeding
Doctors divide the digestive tract at a small ligament where the upper small intestine meets the lower small intestine, roughly at the end of the duodenum. Everything above that point is considered upper GI bleeding. Everything below is lower GI bleeding. This distinction matters because the causes, warning signs, and treatments differ significantly between the two.
Upper GI bleeds tend to produce black, tarry stools (called melena) because blood is partially digested as it travels through the intestines. You might also vomit blood or material that looks like coffee grounds. Lower GI bleeds more often produce bright red or maroon blood in the stool, since the blood has a shorter distance to travel. However, a very fast upper GI bleed can also produce red blood, and a slow lower bleed near the beginning of the colon can produce dark stools. The color is a useful clue, not a definitive answer.
Peptic Ulcers: The Leading Upper GI Cause
Peptic ulcers account for roughly 32% to 36% of upper GI bleeds, down from about 50% in older studies. These are open sores that develop in the lining of the stomach or duodenum, and they bleed when they erode into a blood vessel.
Two things cause most peptic ulcers. The first is infection with a bacterium called H. pylori, which triggers inflammation that damages the protective lining of the stomach. The second is regular use of nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, aspirin, and naproxen. NSAIDs work by blocking an enzyme your body uses to produce prostaglandins, molecules that maintain the protective mucus barrier in your stomach. Without that barrier, stomach acid eats into the tissue underneath. Taking NSAIDs occasionally is unlikely to cause problems for most people, but daily or long-term use raises the risk substantially, especially in older adults or those also taking blood thinners.
Variceal Bleeding and Liver Disease
In people with cirrhosis or other severe liver disease, blood flow through the liver becomes blocked. The portal vein, which normally carries over 1,500 mL of blood per minute through the liver, backs up. The body responds by rerouting blood through smaller veins that weren’t designed to handle that volume, particularly veins in the lower esophagus and upper stomach. These veins swell into fragile, balloon-like structures called varices.
When varices rupture, the bleeding can be massive and life-threatening. Variceal bleeding carries a 10% to 20% mortality rate in the six weeks following the episode, and at least 30% of rebleeding episodes are fatal. The risk of dying is closely tied to how damaged the liver already is. People with early-stage cirrhosis have much better outcomes than those with advanced disease.
Other Upper GI Causes
Beyond ulcers and varices, several other conditions can bleed in the upper digestive tract:
- Gastritis and esophagitis. Inflammation of the stomach lining or esophagus, often from acid reflux, alcohol, or NSAIDs, can cause surface-level bleeding that’s usually slow but persistent.
- Mallory-Weiss tears. These are small tears in the lining where the esophagus meets the stomach, typically caused by forceful vomiting or retching. They’re common after heavy drinking or severe bouts of vomiting.
- Tumors. Cancers of the esophagus or stomach can erode into blood vessels as they grow.
Diverticular Disease: The Leading Lower GI Cause
Diverticular bleeding is the most common cause of lower GI bleeding. Diverticula are small pouches that bulge outward through weak spots in the colon wall. They’re extremely common in people over 60. Most never cause problems, but the blood vessels running alongside these pouches can weaken over time and eventually rupture.
The hallmark of a diverticular bleed is sudden, painless passage of a large amount of red or maroon blood. There’s typically no cramping or abdominal pain beforehand. The good news is that 70% to 80% of diverticular bleeds stop on their own without treatment. The bad news is that up to 38% of people who have one episode will bleed again. Severe diverticular hemorrhage carries a mortality rate of 10% to 20%.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both cause chronic inflammation that can lead to ulcers and bleeding anywhere in the digestive tract. Ulcerative colitis affects the colon and rectum, so it typically causes bloody diarrhea. Crohn’s disease can affect any part of the GI tract but most often involves the small intestine and the beginning of the colon. Bleeding from Crohn’s disease may be less visible, sometimes showing up only as iron deficiency anemia from slow, ongoing blood loss rather than obvious red blood in the stool.
Infections can also inflame the colon and cause bleeding. Bacterial infections from organisms like Salmonella, E. coli, and C. difficile can produce bloody diarrhea that resolves once the infection clears. Reduced blood flow to the colon (ischemic colitis), which is more common in older adults, causes a similar picture.
Colorectal Polyps and Cancer
Polyps are abnormal growths on the inner lining of the colon or rectum. Most are harmless, but some bleed intermittently, and some can develop into cancer over time. Colorectal cancer itself can cause bleeding as the tumor weakens and breaks through the lining of the bowel wall. This bleeding is often slow and invisible to the naked eye, which is why screening tests look for hidden blood in the stool. By the time bleeding becomes noticeable, the cancer may be more advanced.
Hemorrhoids and Anal Fissures
These are the most benign causes of rectal bleeding and also among the most common. Hemorrhoids are swollen blood vessels in or around the anus that bleed when irritated, usually producing bright red blood on toilet paper or in the bowl. Anal fissures are small tears in the lining of the anus, often from passing hard stools, that cause sharp pain and streaks of blood during bowel movements. Neither condition is dangerous on its own, but the bleeding can look alarming, and it’s impossible to tell from appearance alone whether blood is coming from a hemorrhoid or something deeper in the colon.
Angiodysplasia in Older Adults
Angiodysplasia refers to clusters of fragile, abnormally formed blood vessels in the wall of the digestive tract. It’s the most common vascular abnormality in the GI tract and primarily affects people over 60. These malformed vessels are the leading cause of bleeding from the small bowel in older adults.
Unlike many other causes of GI bleeding, angiodysplasia often produces no symptoms at all or causes only slow, painless blood loss. Many people discover the problem only after routine blood work reveals unexplained iron deficiency anemia. When bleeding does occur, it tends to be mild to moderate rather than a sudden large hemorrhage.
Medications That Increase Bleeding Risk
Several categories of medication can either cause GI bleeding directly or make an existing source bleed more heavily. NSAIDs are the most well-known culprits because they damage the stomach’s protective lining. But blood thinners (anticoagulants and antiplatelet drugs) don’t cause ulcers themselves. Instead, they make it harder for your body to stop bleeding once it starts from any source, turning what might have been a minor bleed into a significant one. The combination of NSAIDs with blood thinners is particularly risky.
Aspirin deserves special mention because it acts as both an NSAID and a blood thinner. Even low-dose aspirin taken for heart protection can increase the risk of GI bleeding over time.
Warning Signs of Serious Bleeding
Not all GI bleeding requires emergency care. A streak of blood from a known hemorrhoid is very different from vomiting blood. Signs that suggest significant blood loss include lightheadedness or dizziness, fainting, a rapid heartbeat, and feeling cold or clammy. Black, tarry stools indicate bleeding that has been going on long enough for blood to be digested, and vomiting blood or coffee-ground-like material points to active upper GI bleeding. Any of these symptoms warrants urgent medical evaluation, as significant blood loss can become life-threatening quickly.
Even without dramatic symptoms, persistent or recurrent bleeding matters. Chronic slow blood loss from angiodysplasia, polyps, or inflammatory bowel disease can gradually deplete your iron stores and cause anemia, leaving you fatigued and short of breath over weeks or months. If your stools have changed color or you’re noticing blood intermittently, that pattern is worth investigating even if each individual episode seems minor.